2026 The Battery Cycle

2026

THE

BATTERY

CYCLE

Understanding the heart of BEVs

SPECIAL FEATURE BY

Vado e Torno Edizioni srl, via Brembo 27, 20139 Milan. Phone +39 02 55230950

Printing: RGM srl, Rozzano (MI)

THE

BATTERY

CYCLE

EDITORIAL & CONTENTS

THE PLACE

FOR TRADE

JOURNALISM

The collaboration between Claudius Jehle and

our media is something we are extremely proud

of, and this is neither an exercise in self-congratulation

nor a gesture of flattery. Claudius has a

distinctive ability to simplify everything related to battery

management without ever trivialising it. His contributions

were channelled into an initial series of articles published

on Sustainable Bus website in 2021, which were met with

strong interest and engagement from our readership. Those

articles are now returning in a revised form, updated in

light of the many technological, regulatory, and market

developments that have taken place over the past five years.

Condensed into the special feature you are holding in your

hands—or reading on your e-reader—this work represents

a genuine unicum within trade publishing. This publication

will accompany, and is already accompanying, official

distribution throughout the year at trade fairs and industry

events, while also reaching subscribers not only of Sustainable

Bus but of its sister platforms Sustainable Truck&Van

and Powertrain International, as well. These titles are

connected not only by their shared belonging to the same

editorial group, Vado e Torno Edizioni, but also by a shared

approach to specialist trade journalism that prioritises

not the mere reproduction of product announcements, but

the objective of providing professional readers with solid,

continuously updated tools to understand the evolution of

markets and technologies. In other words, we consider dealing

with rather different industrial sectors by using a similar

and well-proven approach, also relying on qualified and

reliable sources of information, as our strength, in the name

of an integrated and effective trade journalism.

Riccardo Schiavo, Fabio Butturi, Fabrizio Dalle Nogare,

managing editors of Sustainable Bus, Powertrain International and

Sustainable Truck&Van, respectively

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#0 | INTRO

Opening the black box... our way.

Too important to be left unexplained

#1 | NMC,

LFP, LTO

Energy density, safety, lifetime and cost:

every chemistry is a trade-off

#2 | STATE

OF CHARGE

Why knowing how full is a battery is,

is harder than it sounds

#3 | ON

CHARGING

More charging power does not mean

proportionally faster charging...

#4 | BALANCING

INHOMOGENEITIES

One battery, many cells. Imbalance, not

average performance, defines the limits

#5 | STATE

OF HEALTH

Why SoH is the most misunderstood

number in the battery industry

#6 | STRESS

LEVEL

What really shortens battery life?

Degradation is not random: it follows rules

#7 | DATA

TRANSPARENCY

Battery data is powerful, but only if usable.

Ownership, access rights and KPI

TEXTS BY CLAUDIUS JEHLE

3

THE

BATTERY

CYCLE

INTRO

OPENING

THE BLACK BOX

BY CLAUDIUS JEHLE

Battery-electric vehicles are

no longer a niche technology,

they have transitioned from

pilot projects to a widespread

reality. In just a decade, the number of

electric buses worldwide has grown to

nearly 800,000*. In Europe, EVs grew

to 24 per cent of bus and coach registrations**

as they cover over half of the

city bus market in 2025. Electric trucks

above 3.5 tonnes now secure slightly less

than 4 per cent of the European market,

while electrically-chargeable vans (with

GVW up to 3.5 tonnes) have recently

overcome the threshold of 10 per cent

of the overall market, reaching 11.2 per

cent in 2025 on average in Europe**

At this scale, batteries are no longer hidden

components in the background; they

have become central assets that define

a vehicle's availability, cost, safety, and

value.

Bigger batteries, higher stakes

At the same time, the batteries themselves

have changed. Early electric buses

often operated with real-world ranges

of around 200 km and battery packs in

the 240–350 kWh range. Today, typical

depot-charged city buses frequently

reach 280–350 km per charge, enabled

by battery systems exceeding 400 kWh.

Long-range models with 500–600 kWh,

and even higher, are increasingly standard

in tenders. Speaking of trucks, the latest

model introduced in the market boost

around 500-600 km range, with over 600

kWh battery capacity.

This matters, because the battery is no

longer just a component. In many vehicles,

it represents a substantial share of

the total investment and directly determines

availability, operational flexibility,

safety margins, and residual value.

More origins, more complexity

With scale comes diversity. European

fleets today operate vehicles from domestic

and international manufacturers, often

combining different battery chemistries,

charging concepts, and software philosophies

within the same depot. Chinese and

European OEMs follow different design

approaches, and there are still no true

industry standards for how batteries are

sized, operated, monitored, or reported.

The result is mixed fleets with mixed

rules — and rising complexity.

This complexity is real. But it is not unsolvable.

It requires a shift: from treating

Batteries have

become one of

the most important

assets

in public transportation.

They

determine availability,

operating

costs, safety

margins, and

long-term value.

However, there

are still few true

battery experts

in daily fleet

operations. The

Battery Cycle

knowledge series

of article was

created to close

this gap.

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Questions that used to

be theoretical - such as:

what happens after the

warranty? What is the

real remaining value? Is

second life viable? - are

becoming operational

and financial decisions.

Simply trusting a single

number or a yearly

capacity test will no

longer be sufficient.

Five numbers that explain why batteries matter now

The number

What it tells us

~800.000 electric buses worldwide by 2025 Battery-electric buses have

scaled from niche to global

infrastructure within few years.

>50% zero-emission share Batteries are no longer

of new city buses

optional, they are the

in Europe

default for urban fleets.

400 kWh typical battery size Batteries are growing rapidly,

of new city buses

increasing cost, complexity,

and operational impact.

25-30% share of Chinese OEMs Mixed fleets, mixed chemistries,

in the EU e-bus market

mixed software philosophies,

no common standards.

2027 mandatory EU Battery Passport Battery transparency becomes

a regulatory requirement,

not a nice-to-have.

Using batteries to their true potential

Batteries have become one of the most

important assets in public transportation.

They determine availability, operating

costs, safety margins, and long-term value.

However, there are still few true batthe

battery as a black box to treating it as

a managed asset.

External pressure is increasing as well.

New regulations, such as the EU Battery

Regulation and the upcoming Battery

Passport, will require transparency across

the battery lifecycle. Questions that used

* Data from IEA (https://www.iea.org/data-and-statistics/data-tools/global-ev-data-explorer)

** Data from ACEA

to be theoretical - such as: what happens

after the warranty? What is the real remaining

value? Is second life viable?

- are becoming operational and financial

decisions. Simply trusting a single

number or a yearly capacity test will no

longer be sufficient.

tery experts in daily fleet operations.

The Battery Cycle knowledge series of

article was created to close this gap.

Rather than oversimplifying complex

electrochemistry, it makes the underlying

mechanisms understandable, such as how

state of charge, state of health, degradation,

balancing, charging strategies, and

stress interact in real vehicles under real

operating conditions.

What started as technical discussions

grew into a shared belief that knowledge

is the most powerful tool operators have

in the transition to electric mobility.

Every battery must be used to its true potential. Yet

even today, too many batteries are replaced

too early, misunderstood, or trusted blindly because they sit inside a

black box. The Battery Cycle is our way of opening that box.

Together with Sustainable Bus, a partner we’ve worked with almost

from the beginning, we aim to share knowledge, spark curiosity,

nd encourage operators to ask better questions. Not to turn anyone

into a battery scientist, but to make sure decisions about such critical

critical assets are based on understanding, not assumptions. If this

series makes you rethink your fleet, your data, or your strategy,

we encourage you to start the conversation. Real progress begins

with asking better questions: contact@volytica.com.

Claudius Jehle, CEO & Co-Founder, volytica diagnostics

5

THE

BATTERY

CYCLE

#1

NMC, LFP, LTO. WHAT'S

THE DIFFERENCE?

Spotlight on key lithium-ion battery chemistries—NMC,

LFP, and LTO—and their impact on energy density,

lifespan, and safety. An analysis at the intersection of

technology and market trends

What do NMC, LFP, LTO

and the like mean, what

properties do they share

and what are their individual

strengths and weaknesses?

In a fully charged battery cell, many

billions of lithium ions are trapped in a

porous reservoir called the anode, typically

a foil or sheet of some particular

material we will investigate later. They

are embedded within the material structure

like parachutists in an airplane, excited

to leave their spot and to travel to

the opposite reservoir to rest again, called

cathode. Their journey begins when the

anode and cathode are connected – e.g.,

by switching on the electric motor. The

lithium ions leave the anode, while each

releases an electron that travels the external

circuit to power the motor as electric

current.

Once released, each forces its way

through an intermediate medium. This

layer must allow swift transport of ions

while keeping the anode and cathode

safely separated by a short distance, to

prevent short circuits.

This contradictory requirement, permeable,

light and thin, yet insulating and

robust, has long caused headaches for

engineers. It also constitutes “dead”

weight, reducing capacity per volume and

weight. As of 2025, low-viscosity chemical

liquids or polymers ('electrolyte') submerged

in a matrix ('separator') remain

state of the art.

We will cover solid-state batteries, where

the electrolyte and separator are replaced

by a solid layer, in a future article. They

promise improvements in safety and

density but pose new engineering chal-

6

lenges. And let’s

be clear: “solid-state

will replace

Liion”

is like

saying “bananas

will replace

fruit” - solid-state

is still a type of Liion

battery.

After making their

way through the

electrolyte, they deploy

their parachute,

slowing down before

crowding the entries

to the cathode material.

One-by-one, they

find energetically advantageous

positions

within the cathode lattice

by migrating slowly into

the material.

It becomes clear that the

seemingly trivial process –

which similarly, but not identically,

happens while charging

– involves interaction of the ions

with a multitude of different materials,

at different speeds and in different

processes.

Just Imagine alone the obvious effect

of temperature on the viscosity of the

electrolyte and thus the process speed of

this crucial middle passage Add to that

the mechanical expansion stress of forcing

billions of bulky ions into a tightly

packed material structures. This has

serious implications for battery health,

which we will examine in a future article

Today the overwhelming

majority of cells used

in stationary and most

(heavy-duty) mobile applications

are LFP-based.

Why? Because China,

which can be viewed as

the sole and only producer

of this type of Li-ion

technology, has brought

production to an unprecedented

scale, and the

price down to ever more

affordable dimensions.

on degradation.

Now, to keep it simple, we can say that

the anode reservoir is almost always

made of graphite, plus a myriad of super

secret additive and variations in manufacturing

and processing. Thats why we

typically just don’t mention it in communications.

An exception is LTO, which we

will briefly mention later.

What is NMC about?

When it comes to the cathode however,

the topic becomes more complicated.

Cathode materials using compositions

of nickel (Ni), manganese (Mn),

the highly disputed

cobalt (Co) and

sometimes aluminum

(Al),

all in greatly

varying compositions

and

again with

undisclosed

secret additives

and recipes,

are simply

referred to as

NMC or NCA.

Such cathodes

typically have

the following distinguished

properties:

higher energy

density (i.e. many

seats for parachuting

ions per volume),

lower lifetime

(densely packed testosterone

comes at a price),

lower safety margins, higher price.

Particularly the top energy density property,

that can moreover be adjusted by

increasing the nickel-content* (and unfortunately

equally decreasing lifetime

expectation), today make NMC cells the

chemistry of choice for hi-performance,

long-range and hi-end applications.

* if Ni, Mn and Co are used in equal

proportions, one says “NMC111”. There is

a trend in automotive to go for 8 times Ni

and equal parts Mn+Co, yielding NMC811

– long range, low lifetime.

As always in life, there

ain’t no such thing as a

free lunch. Li-Ion batteries

share a common baseline

mechanism of ion transport

through media, and

an impressive array of

different combinations of

different such media push

performance into one

direction or the other.

sis, are cathodes made of LiFePO4 (Lithium

Iron-Phosphate, LFP) – renowned for

being: less energy dense than NMC, quite

robust, rather economical (abundant materials,

no Ni and particularly Co).

Despite increasing popularity of LFPbased

systems, the lower price (facilitated

by mass production in China)

cannot fully compensate for the lower

energy density – and LFP cells feature

a very unique trait: over a very broad

range of SOC (state of charge), their

voltage does virtually not change. This

is good for power electronics, but as we

will see in future articles, this makes

SOC determination and cell balancing a

downright frustrating job, making LFP

applications generally prone to unexpected

operation problems and downtimes,

further aggravating the lower

energy density.

It’s like filling up a wine decanter: the

filling level in the bulgy part of the vessel

won’t change much even if you pour the

second bottle of juicy red wine. A horror

to the electronics, BMS and operators, as

there will inevitably be estimation mistakes

on how much energy there actually

really is available at the very moment. In

practice, this not only limits the intrinsic

lower energy density further, it leads

to erratic behaviour, and this expensive

technology is hoften not used to its true

potential.

Last but not least… LTO

Now, last but not least, there’s at least

one exotic around: cells with an anode

not made of graphite, but Li-Titanate

(Lithium Titanoxid, LTO), often paired

with an LFP cathode. Such LFP-LTO

cells are often incorrectly just referred to

as LTO, embezzling the decisive cathode

material. These systems are typically 'the

NMC’s nemesis: LFP!

However… the major antipole, or nemespecial

force': extremely robust ('military

grade'), safety and longevity-wise

(10-30x LFP or NMC), very low energy

density (lower than long-forgotten vintage

batteries like NiMH), quite costly

(at least 50-150 percent more expensive

than NMC).

Punchline: like animals in a jungle

Some of us had combinatory math in

school: if there are myriads of ways to

build, and millions of additives to add,

to an anode, cathode, separator and electrode,

there are billions of ways to build

a Li-Ion battery – even if the result might

still fall in one of the NMC/LFP/… categories.

Any and all generalizations (“NMC is

like this and LFP like that”) will have to

limit themselves to a few general bullet

points, as the ones given above. There is

just such a terrific variety of possibilities

for the cell manufacturer to tune the performance

or to tease out particular properties,

that generalizations are often ‘thin

ice’.

To sum up: as always in life, there ain’t

no such thing as a free lunch. Li ion batteries

share a common baseline mechanism

of ion transport through media, and

an impressive array of different combinations

of different such media push performance

into one direction or the other.

Improvements are made here and there,

but a giant leap in energy density without

negative effects on, say, lifetime, cannot

realistically be expected. Rules of thumb

exist, but the most expensive wearing

part in an e-vehicle does and will remain

to require high attention!

Hands-on tips

Know your chemistry and design your

operations around it.

If you're running LFP: watch SoC accuracy

and plan maintenance around

potential balancing issues.

If you're using NMC: monitor SoH

closely to avoid range loss surprises.

If you're considering LTO: weigh cost

and space vs. robustness.

Different chemistries require different

KPIs. Make sure your analytics

speak the right dialect.

7

THE

BATTERY

CYCLE

#2

UNDERSTANDING

STATE OF CHARGE

An in-depth look at why battery management systems struggle to

accurately determine State of Charge, examining voltage behavior,

chemistry-specific characteristics, and operational constraints.

Electrochemistry, BMS logic, and performance limits come into play

It is surprisingly difficult for the onboard

electronics (which we will refer to as

BMS, battery management system) to

determine the State of Charge (SoC)

even if these systems have signals like cell

voltages, currents and temperatures directly

at hand. Building on the previous article

about battery chemistries, we will stay figurative

and use the same ironical, but very

helpful, analogy of parachuters for ions,

congesting at the exit of their vessel (aka

anode) during discharge, their adventurous

and tedious travel through electrolyte and

separator, their crowding during landing and

finally slow migration into and settling in the

opposite electrode (aka cathode).

State of charge: how full is it?

Unfortunately, our microscopic (i.e. on

atom/ion level) parachuter analogy becomes

clumsy when explaining a macroscopic effect

like SoC – so think of a glass with a

certain content (trillions of parachuters). The

maximum possible content is the capacity.

The SoC is simply content per capacity, i.e.

150ml in a 300ml glass results in 50% SoC.

Of course it doesn’t stop here, unfortunately.

The only signals the BMS can really

measure, in the very sense of the word, are

the cell voltages and the current flowing

through the cells (okay, and temperature, but

that’s not needed for now). Yes, it doesn’t

'measure' SoC, it’s estimating it.

In the analogy of a glass, the cell voltage

is synonymous for the

current filling height. If

you can accurately measure

this, and you have a

translation table to convert

height into content, you

should be good. That’s

8

Drawing by Friederike Dippon,

Volytica Diagnostics

exactly how a BMS determines SoC…

…once in a while, at least, because most

BMS cannot do this continuously: there is

something that I call 'foam'*. The liquid in

the glass is not water, it’s foamy beer. When

drafting, gulping or shaking (i.e. when using

the battery, or charging it), a layer of foam

builds up, making it difficult even for the

trained eye to measure filling height to the

fraction of millimetres. This phenomenon

and the characteristics of this 'foamy voltage'

(mostly called 'overpotentials')

are highly dependent

on chemistry, immediate

past current flow, temperature

and many other nasty

things.

The foam/overpotentials

The SoC is simply content

per capacity, i.e. 150ml

in a 300ml glass results

in 50% SoC. Of course it

doesn’t stop here, unfortunately.

The only signals

the BMS can really measure,

in the very sense

of the word, are the cell

voltages and the current

flowing through the cells

(okay, and temperature...).

Yes, it doesn’t 'measure'

SoC, it’s estimating it.

slowly decay, just like on a beer. The BMS

frequently needs phases of idle and rest of

typically 30-60 minutes, to be able to reset

the SoC. In the busy times in-between, the

only way the BMS can somehow keep track

of SoC is to count how much beer (=current)

went in and out and adding/subtracting

it from the last known reset value. This is

called Coulomb Counting, and the longer

the time from last reset, the farther off this

tally taking will be**.

Why is measurement so hard?

Guess what, we’re still not done. So not

only needs the BMS frequent and prolonged

phases of rest to reset, and in the meantime

roughly tracks SoC by tally taking, the

shape of the glass, the mentioned ‘transla-

Fun fact

Ever wondered why fast-charging is

'fast' only until ca. 80%?

It takes 45 minutes to charge up

to 80%, but another 45 minutes to

charge the last 20%.

The core reason is not artificial manipulation

by the charging system or the

vehicle to keep degradation low.

Have you ever poured a glass of beer

from a tap?

80% in no time, but for the last 20%

you need to reduce the current to keep

the foam from spilling over.

No joke. The ions cannot migrate

into the electrode quickly enough, so

charging current must be reduced to

allow the congestion to dissolve.

The fuller the electrode, the slower the

migration, the more current reduction.

tion table’ is yet another obstacle.

In the illustration, a cylindrical glass is

shown. Easy game: 2x as much filling

height means 2x the SoC, absolutely linear

relationship. Sadly, that’s not the case for Li

ion batteries: as alluded already in the last

* Disclaimer: there is no real foam, again just an

analogy. For the geekier readers: The congestion

of ions at the gates to the electrodes, i.e.

them slowing down before entering the anode

or cathode, and their then ensuing slow and

tedious migration into that solid material, is

causing the macroscopic effect of the settling

voltages (foam) after current was flowing. It’s

called the overpotential voltages that build up

when current is flowing and slowly reduce after

this has stopped. Exactly like drafting a beer.

** There are two fundamental reasons why

Coulomb Counting is inaccurate: firstly,

because every current sensor has tiny offsets

and biases; if you keep adding them up, they accumulate

and will cause the assessment to 'drift

away'. I bet when you drink a large glass of beer

and had to guess the exact amount of remaining

beer just by counting your sips, you’d fail. At

least I would. And secondly, the BMS needs

the exact maximum capacity (glass size), which

shrinks as the battery ages (we will cover than

in the article about SoH), which is an even more

challenging task, and BMS-SoH is notoriously

wrong.

Easy game: 2x as much

filling height means 2x the

SoC, absolutely linear relationship.

Sadly, that’s not

the case for Li ion batteries:

as alluded already in the

last article, depending on

chemistry, this (and other

properties) of cells varies

significantly. The 'glass

shapes' are highly irregular

and manifold across chemistries.

article, depending on chemistry, this (and

other properties) of cells varies significantly.

The 'glass shapes' are highly irregular and

manifold across chemistries.

To get images into your head, LFP looks

like a broad, bellied, bulbous wine decanter

and NMC has rather an elongated candy-shape

(narrow, fairly straight, narrow).

This lookup-table to translate height (voltage)

to content is determined once by the

manufacturer in a lab and programmed into

the BMS; it is called the 'open-circuit voltage

characteristics', or also OCV curve.

One battery, one number?

The main challenge, as one can easily imagine,

comes during operation: 5mm of

height in the belly-part can be a good 20

or 30% of total liquid content! In fact, LFP

is so broad that between ca. 20 to 90% the

voltage virtually doesn’t change. The slightest

measurement error can result in extreme

SoC errors. And then there is also this foam!

Imagine the BMS of an LFP battery joyfully

finds itself one of those deadly needed idle

periods and prepares for a reset – it waits

until foam has settled (hoping that the asset

remains at rest) but then realizes that it

came to rest in the bellied part! The voltage

measurement could equally translate to 20%

or 40% SoC – an ordeal!

I will wrap things up in a moment, but for

the full truth, you need to bear with me for

another moment. What you have learnt so

far is that, particularly for LFP, accurate

SoC determination is sometimes sheer luck.

The often heard “…but Mr. Jehle, the BMS

has full access to everything in microsecond

resolution” is fair but doesn’t change

physics.

So far, we spoke specifically about how

the BMS determines cell-SoC. But you are

interested in the SoC of the whole battery

system. Well, the (Master-)BMS has the

thankless task to come up with one single

representative SoC for a flock of hundreds

or thousands of individual cell values. And

yes, they differ. Not in theory, but in practise.

For reasons we will dedicate at least one

article to, this 'disbalance' (also called imbalance

or asymmetry) causes a challenge.

The BMS cannot do the obvious and take

an average of all cell SoC, because when

discharging, the cells below average will

be empty first and demand a stop at >0%

SoC, and vice-versa when charging. So oftentimes

there is sophisticated logic at play

when the BMS performs this task, and believe

me, sometimes making things worse.

So, what?

Wait, what does this mean in practice? One

thing is universal: I estimate the average

SoC error to be somewhere between 5-10%,

reaching up to 10-20% in e.g. low-cost systems,

high-duty (few breaks) applications

and especially LFP/LTO systems. This is

significant – a battery that’s off by 10% is,

basically, 10% oversized!

Inaccurate SoC causes batteries to be oversized,

operations to be erratic, assets not

used to their true potential, money being

wasted and frustration rising.

Achieve an accurate SoC via:

1. High-quality products (e.g. hi-accuracy

voltage and current sensors)

2. Periodic and sufficiently long breaks (for

resetting and, consequently, balancing)

3. Particularly for LFP/LTO applications:

these resetting and balancing breaks

should be at >80% or <20% SOC, to be

outside the 'belly region', making it tremendously

easier for the BMS to function, e.g.

after charging.

4. Make use of centralized battery monitoring

with advanced SoC recalculation

and warning functionalities, which will help

you identify 'off' assets, schedule balancing

and maintenance and single out faulty assets.

But: high standstill SoC is one of the most

detrimental degradation facilitators. Depending

on the cell chemistry, there is

a sweet spot somewhere between 80-

100%. Ask your supplier or, rather, an

independent expert such as diagnostics

providers or academic labs for directions.

9

THE

BATTERY

CYCLE

#3

CHARGING BATTERIES,

FOOD FOR MINDS

An examination of charging strategies for lithium-ion

batteries, from depot to opportunity charging, highlighting

CCCV behavior, power scaling limits, and chemistrydependent

effects

Picturing batteries like bulbous

wine decanters filled with foamy

beer, as we did in the previous

article, greatly helps to understand

everyday problems with SoC estimation,

balancing problems and sudden

standstills. But this analogy also greatly

Assume you would just

stop charging when the

voltage first hits the top

– foam would settle and,

somewhat surprisingly,

you would find your glass

only 85% full after some

10 to 30 minutes. So it is

exactly like drafting a beer:

CC beer flow until the foam

hits the rim, then reduction

of beer flow, keeping the

total height constant (CV).

helps to understand typical questions

around charging of batteries, which we

will address in this article.

It’ll be the place where to explain –

among many other things – how and

why doubling charging power cannot cut

charging time half, at least if one intends

to charge until full.

Let’s start with depot charging setups, i.e.

with buses or trucks with rather large batteries

(>300 kWh) that are meant to serve

a route all day with no or only few top-up

charges. Typically, one wants the battery

to be at 100% in the morning and to end

with some reserve at the end of the day.

Is depot charging a wise choice?

But charging a battery to 100% before

the day starts is not as straightforward as

one might think. The charger first 'negotiates'

the upcoming charging process with

the vehicle via some sort of dedicated

communication protocol, involving an

agreement on – first and foremost – the

planned charging power that the charger

is able to give, but also that the battery is

capable of receiving. We limit ourselves

to the battery part here, because one

could write endless articles on this delicate

and fragile communication part. In

particular, we are explaining the situation

for well-sized and not super-oversized

batteries; for oversized batteries, the below

explained CV-part typically does not

affect charging so much.

Typically, charging commences with the

agreed current in a constant flow – this

phase is thus called constant current

phase or simply CC phase (see graph on

the right). In the beer analogy, the glass

constantly fills up, and the filling height

increases; both the beer rises, and foam

starts building – electrochemically speaking,

'overpotentials' on the measurable

voltage start building up (if you read the

other articles before: this is due to the

migration of the bulky ions into the electrodes

taking some effort, if you will, and

this is macroscopically visible as reversible

voltage build up, slowly vanishing

again after charging stops, just like foam

on beer…).

Once the total filling height, i.e. beer plus

foam, approaches the rim of the glass, i.e.

the battery voltage reaches the absolute

maximum admissible voltage, charger

and battery quickly start renegotiating a

new, smoothly changing current profile

that keeps the total filling height just constant

at the top – the constant voltage or

CV phase. What in essence happens is

that stepwise reducing the current will let

the foamy part decrease, while the liquid

part can continue to rise. This process in

total is often called 'CCCV charging'.

Assume you would just stop charging

when the voltage first hits the top – foam

would settle and, somewhat surprisingly,

you would find your glass only 85% full

after some 10 to 30 minutes. So it is exactly

like drafting a beer: CC beer flow

until the foam hits the rim, then reduction

of beer flow, keeping the total height constant

(CV), only changing the liquid-tofoam

ratio.

Doubling or tripling charging power leads

to more overpotentials ('foam'), which

reduces energy intake compared to a

base-scenario: tripling power can result in

less than doubling effective energy intake!

[Source***]

OEMs and manufacturers

distinguish between the

SoC that is communicated

(often 'operational SoC')

and the 'real', internally

calculated one ('technical

SoC'), which is obviously

particularly relevant for

oversized batteries where

the usable, operational net

capacity differs significantly

from the technically

available capacity.

bling charging power cannot cut charging

time half, at least if one intends to charge

full. As a matter of fact,

the CC part is sped up, but

also not by the factor 2, as

the foam rises much faster,

so the CV part becomes

even more pronounced. In

the example, there is even

no CV part reached after

~900s (it would come

later), but by doubling

the power, CV is reached

after ~570s, reducing the

energy intake by 24%.

Tripling the power obviously

brings the system

into CV right away (no

CC observable), resulting

in a less than double energy

intake! Needless to say,

this is just an example and

might not be applicable to

your specific assets. Oversized

batteries – and this is very true for

LFP-based assets – don’t show this effect

as pronounced as explained here (as the

CV-part might not even be reached, i.e.

you are drafting ca. 1 liter of beer into a

1.5 liter glass: you will hardly produce

enough foam to hit the rim).

Learning? 3x charging power doesn’t

necessarily reduce charging time by 3!

It can easily be that the CV part takes just

as long as the CC part. To get the first

80% in can take just as long as to get the

last 20% in! Observe it in your monitoring

systems: first, SoC rises linearly, then

it slowly decreases until it rests at 100%.

If you cannot observe this in your data

when approaching 100%, the very likely

reason is that the SoC that you see is not

Getting to know the implications...

What are the implications? Look at the

simulation results in the diagram – one

straightforward consequence is that douthe

actual, the 'real' SoC. OEMs and manufacturers

distinguish between the SoC

that is communicated (often 'operational

SoC') and the 'real', internally calculated

one ('technical SoC'), which is obviously

particularly relevant for oversized batteries

where the usable, operational net

capacity differs significantly from the

technically available capacity.

The operational SoC is adapted to e.g.

show 100% when the real is only at

~90%. Without going too much into detail,

there are good reasons to do this: (a)

to have some reserves (to accommodate

the known estimation errors, cf. the last

article), (b) to save the battery (too high/

low SoCs are often detrimental to health)

and (c) to emulate a seemingly uniform

behavior over the whole lifetime by unlocking

the reserves over the lifetime. So,

looking at (c), if you do not observe this

charging behavior now, you might well

observe it later – bear in mind that the

depot charging time of the vehicle might

Hands-on tips

Don’t just request access to battery

data, ask for the right signals in the

right resolution.

Parameters like SoC, SoH, current,

voltage, and temperature should be

logged at high enough frequency to

reflect real charging behavior.

11

THE

BATTERY

CYCLE

#3

increase in the future!

What about opportunity charging?

With opportunity charging along the

route, there are other challenges. One

typically does not charge to 100% (only

in the morning, as above) but repeatedly

top up a smaller battery (<200 kWh). The

foam hardly hits the rim, so no tenacious

CV phase, only CC. More like topping

up a glass of water, or red wine, now and

then. For speedy charging interruptions,

the power is typically quite high. So although

there is no CV part that could

'choke' the fast charging, the high pulses

can result in resistance-induced voltage

spikes (here the glass & liquid analogy

fails a bit – maybe think of sparkling

wine: you pour it in, the foam extremely

quickly rises very high and decays in

virtually only seconds). As the resistance

increases due to degradation, these spikes

increase likewise. Take-away message:

opportunity charging has beneficial characteristics,

at least charging timewise,

but can suffer from degradation induced

At a glance

1. Li ion batteries exhibit a curious

effect: the amount of energy chargeable

in a certain time depends on the

charging power, e.g. doubling charging

power does not necessarily cut the

charging time in half.

2. The SoC you see (“operational/ net

SoC”) is most often not the real SoC,

for good reason.

3. In depot charging applications or

anywhere, where a battery is charged

to 100% (really 100%), and where the

battery is not heavily oversized, some

form of CCCV charging is employed.

4. The peculiar pattern of CCCV leads

to the that behavior that doubling or

tripling charging power does not cut

the time half or by a third.

5. Opportunity charging is mostly CC,

so it is not susceptible to this pattern,

but is susceptible to rising resistances

due to

12

Opportunity charging is

like topping up a glass of

water, or red wine, now and

then. Take-away message:

opportunity charging has

beneficial characteristics, at

least charging timewise, but

can suffer from degradation

induced charging time

increases.

charging time increases.

Finally: inductive charging…

One remark on inductive charging, where

the physical plugging for a galvanic connection

is replaced by wireless over-theair

energy transmission. It’s a fabulous

idea, as there would be virtually not

positioning and manual handling efforts

anymore. One could even charge while

driving. But physics kick in: The ratio of

energy that reaches the vehicle vs. what is

lost due to peculiar induction inefficiencies

– plainly: that is converted into heating

the environment – chiefly depends on

(a) the distance between the receiver and

the sender and (b) the frequency of the alternating

electro-magnetic induction field.

For inductive charging not to become

uneconomically unbearable, you either

have to (a) reduce the distance between

sender/receiver to only millimeters and/or

(b) increase the frequency and thus coil

size and thus weight. Ideally, a (several

hundred kilograms!) copper coil should

be located millimeters away from the

sender. This proved, and proves, to be

technologically so challenging that until

today, no economically viable and scalable

solution was found, and close to

* i.e. OCV characteristics, see article #3

** i.e. the overpotential/dynamic voltages

*** Simulation (LFP, SOC0=50%, 30°C

env) results from: Bunzel, A.; Morawietz,

L.; Ufert, M.: Technologische und ökonomische

Bewertung der Elektrifizierung

von ÖPNV-Busflotten im Werkstatt- und

Betriebshofbereich. Fachtagung „Werkstattund

Betriebshofkonzepte für Elektrobusse“,

Dresden, 11.-12.10.2018.

Overhead catenary charging

Overhead catenary charging (like in

trolley buses or some e-truck application

ideas) deserves also a mention:

next to the obvious infrastructural

challenges of maintaining a catenary

network, a real challenge lies in

the electrical setup of the vehicles:

most road vehicles have rubber tires.

They don’t conduct electrical current.

Hence rubber-tire vehicles with catenary

need two overhead wires (adding

to the infrastructure complexity vs.

metal-tire trains). The real 'but' however

is safety: In a metal wheel-setup,

an electric fault (such as: a live wire

touching the vehicle frame and setting

it under high voltage) would be immediately

diverted to the ground via the

wheels; the danger of metal frames

or vehicle body being 'under voltage'

is very low. But in rubber tire assets,

an electric fault might set the whole

vehicle under voltage, and a passenger

touching the body might get an

electric shock. The result is that the

electrical setup of such vehicles adds

complexity and costs, making it more

challenging economically.

Charging dynamics with sparkling wine vs.

beer analogy. (Visual designed by volytica.)

100% of all batteries are charged with

direct connections.

At the end, we encourage everybody to

openly discuss the options and implications

of different charging philosophies

with their OEM partners. It is very helpful

to ask for maximum transparency in

the basic battery & charger signals and

even for periodic information about the

degradation, as we already now learned

now how degradation affects every aspect

of a battery.

THE

BATTERY

CYCLE

#4

As 'power' is 'voltage x

current', doubling the

voltage (by putting cells in

series) cuts the necessary

current half, and as cable

diameters is mainly determined

by maximal current,

raising system voltage by

putting cells in series basically

helps to reduce cable

size, weight and costs. If

we want more capacity,

we add whole strings in

parallel. But new problems

occur: disbalance, inhomogeneity,

inaccessible

energy and the necessity

for balancing.

Credit picture: Forsee Power

BALANCING

INHOMOGENEITIES

An examination of battery cell imbalance in lithium-ion

systems, explaining how series-connected packs amplify

small cell differences and how the BMS monitors voltages,

enforces safety limits, and performs balancing

A

battery is not one cell, although

we might want to

reduce it to that in everyday

conversation. Ideally, it

should behave like it, but as always,

things are more complex in reality.

In a battery system, dozens or typically

hundreds of single cells are connected

in series (that is, the plus of one is

connected to the minus of the next and

so on), forming a string. Each additional

cell raises the total system voltage.

As 'power' is 'voltage x current',

doubling the voltage (by putting cells

in series) cuts the necessary current

13

THE

BATTERY

CYCLE

Hands-on tips

1. Plan for passive balancing. After

charging to 100 %, follow the OEM's

recommendations for idle time to allow

the BMS to balance the cells effectively.

2. Request granular battery diagnostic

data. Ask for SoC spread, cell voltages,

and deviation metrics, ideally

at cell level, or at least per module or

string.

3. Specify it in tenders. Require a defined

balancing strategy, a max cell

imbalance, and long-term access to

raw battery data.

half, and as cable diameters is mainly

determined by maximal current, raising

system voltage by putting cells in

series basically helps to reduce cable

size, weight and costs. As simple as

that*. If we want more capacity, we

add whole strings in parallel.

But by this simple trick new problems

occur: disbalance, inhomogeneity, inaccessible

energy and the necessity for

balancing.

Inhomogeneity: a thought experiment

If we depart from a system where all cells

are the same, we run into serious problems

that we will explain here. As usual,

we start with simple analogies that we

(almost) perfectly suitable for everyday

conversations. Consider a single string

system (works perfectly the same with

multistring systems) and figure it – attention:

metaphor – as a cascade of the

already introduced glasses, discharging

into one another.

Would you agree that, as long there is no

leak, the amount of liquid going in on the

In very simple words, the

balancing circuit in the

Battery Management System

must take care that all

cells have the same SoC

(filling content), ideally

at all times, so that the

weakest link doesn’t limit

overall performance in the

described way.

top completely passes unchanged through

each and every single cell?

Analogy of an ideal & homogeneous

system – all cells are identical and their

SoC reduces at the same speed, uniformly

depleting them. For the sake of simplicity,

we omitted that fact that there should be

overpotentials ('foam').

So, if now all have exactly the same capacity

(SoH, later articles), and all start at

the same filling height (voltage), then all of

them have the same starting content (SoC).

If current flows through the cascade, each

cell in this perfect example is being filled/

emptied by the exact same amount of liquid.

If you keep discharging them, they all

approach 0% SoC simultaneously**. Vice

versa, if you keep filling them up, they all

will be 100% full at the same time.

Without balancing, a system with inhomogeneous

cell SoCs might sooner or

later encounter 'sudden depletion events',

having a safety circuit shutting the whole

system down when the 45% 'lemon' cell

hits 0% before all the others.

The battery appears suddenly empty. The

consequence is system emergency shutdown

we sometimes call a Sudden Depletion

Event (SDE) – a vehicle that until

minutes ago displayed a substantial and

unproblematic SoC suddenly stops and

the SoC drops to 0%. By the way: if you

followed all articles so far, you should

have a feeling why especially LFP is

prone to that effect.

Balancing is key!

What is balancing now

doing? In very simple

terms: the balancing circuit

in the Battery Management

System must

take care that all cells

have the same SoC (filling

content), ideally at all

times, so that the weakest

link doesn’t limit

overall performance in

the described way. The

simplest, but also the

most common, approach

is often called passive

balancing, and it seems

wasteful: a master logic

determines the SoC of

the 'lemon' (or lemons) and discharges

all other cells****. Yes, in above’s example,

a passive balancing system would

start leaking ca. 5% from all the 99 good

cells, almost 'bleeding' them out*****.

This energy is lost.

There are also active balancing systems

in place, which are way more sophisticated

and themselves prone to failures

due to their complexity, and more invest

intensive. The result is also that all 100

cells would end up at the same SoC, but

by actively shifting excess charge from

the 99 others to back up the lemon,

no charge is lost in the process.

When does balancing happen?

In large and very busy stationary

systems, active balancing is gaining

some foothold. However, the

simplicity and robustness of passive

systems still make them the

means of choice in almost all applications,

bus and truck included. So, to

keep it simple, the following is true only

for passive systems.

As said, a balancing system must first

determine the SoC of every cell. As you

learned in the past article, this is particularly

difficult for electronics in dynamic

or 'foamy' situations, i.e. during operation

or charging, and for battery chemistries

with flat OCV curves – like LFP. So the

balancing system will want long, 'relaxed'

idle periods without any power flowing,

and is even more happy if the SoC is

very high, as the 'glass shape' of most

chemistries allows for an accurate voltage-to-SoC-conversion

when almost full

(or empty, btw). The longer the idling, the

more disbalance can be 'heated away' by

leaking.

Now you understand why OEMs recommend

an extended idling phase directly

after charging to 100%? Exactly: idling

* Many readers might object – the power

electronics perform better at higher voltages,

too! Correct, but mainly due to comparable

reasons, i.e. current density.

** Note that for the moment we think of the

process to be extremely quick, i.e. that the

resistance of the cells in extremely low, and

again the same for each and every cell. This is

of course not the case in reality.

*** Actually, it is 49.95% (1×45% +

99×50%), but never is a SoC given with 2

decimals.

**** In fact, every cell with SoC > lemon is

discharged using small heating resistors.

****** In fact, this process is sometimes

called 'bleeding', the circuit using 'bleeding

resistors'

at high SoC facilitates balancing – the

longer the better.

Reasons for inhomogeneity

We have not answered the questions why

at all the cells don’t behave the same.

We brought up the example where one

cell is slightly emptier than the others

(i.e. SoC). The reality is very complex,

with many interactions.

To highlight one very

mean -- because self-enforcing

-- reason: due to

manufacturing imperfections,

uneven cooling/

heating flows, welding

issues, electronics imperfections

etc. some

cells degrade slightly

faster than others – really

only slightly – leading

to a spread in capacity

(SoH). When continuously

charged and discharged,

the differences

in SoH will lead to the

SoCs of all cells starting

to diverge in a manner

comparable to the first

thought example.

To make things worse, a cell that is

slightly smaller than the others experience

more current per capacity (C-rate),

even increasing degradation stress, and

making it even smaller than the rest.

Also, it is likely to have slightly increased

resistance, leading to increased

temperature. Also, the SoC window is

affected. A 'pre-lemon cell' is thus likely

to continue an irreversible downward

spiral.

While the balancing system is always

trying to level out SoC, it cannot affect

the irreversible SoH spread. This

can be mitigated by a balancing system

for some months or years, at least the

operator will not directly feel the ongoing

process. But at one point in time,

balancing times would take so long to

completely level everything out, that operation

would be severely affected. Or,

if it was kept still too short, the risk for

SDEs would significantly increase.

So, what?

OEMs and manufacturers are very aware

of what we wrote above, and the systems

are typically well-equipped with

While the balancing system

is always trying to level out

SoC, it cannot affect the

irreversible SoH spread.

This can be mitigated by a

balancing system for some

months or years. But at

one point in time, balancing

times would take so long to

completely level everything

out, that operation would be

severely affected.

the right balancing setups. It is however

instrumental that customers are aware of

the (dis)balance and risk of system underperformance

due to it for each asset, and

that a proper balancing strategy and adherence

to the OEM’s recommendations

are met.

At a glance

1. Battery systems are complex.

They consist of hundreds of cells

pretending to be one: if they don’t,

one cannot extract all seemingly

available energy.

2. In worst-case situations, unbalanced

systems can suddenly appear

to 'self-deplete' in Sudden Depletion

Events, leading to sudden, costly,

and potentially dangerous en-route

standstills.

3. Balancing requires time and specific

conditions. It most often occurs

during extended idle phases after a

full charge.

4. The reasons for imbalance and inhomogeneity

are complex. A pre-imbalance

can lead to self-reinforcing

internal degradation, which worsens

the problem.

5. It is advisable to adhere to OEM

recommendations and independently

keep track of inhomogeneity (reversible

and irreversible) and balancing

effectiveness.

15

THE

BATTERY

CYCLE

#5

STATE OF HEALTH

What does State of Health actually measure, and who

defines it? Exploring how BMS software, capacity buffers and

degradation tracking shape the reported SoH, revealing the gap

between technical battery condition and the available capacity

The term State of Health or SoH is

easily the most controversial term

in the battery industry. No other –

spoiler: un- or ill-defined – term is

used so liberally and in so many contexts

and for so many purposes as SoH. If we

could turn back time, we would try to get

rid of it altogether, hence we often tend to

put it in quotes. For reference: almost all

financial warranty risk of the whole industry

is tied to SoH, but the industry lacks a

universal definition of this: unprecedented

in the younger industrial history.

But as a matter of fact, it is there, so what

is it, and what is it not? Let’s find out.

To set the stage, we will use SoH broadly

speaking as a relative (i.e. in %) measure

At a glance

The capacity you experience in daily

operations and during capacity

tests is primarily software-defined.

Depending on the implemented BMS

strategy, it is not necessarily affected

by degradation.

The BMS defines th e capacity you get

(and legend has it, in rare cases does

not want to) tell the entire truth about

the full technical capacity.

BMS systems follow shades of gray between

the two fundamental strategies:

'keep or eat the buffer'. The latter leaves

a 'blind' operator with the impression of

no degradation, an infinite lifetime, and

perfect value retention.

We strongly advocate replacing the notion

of 'SoH low = end of life' with 'end

of warranty'. To determine the true 'end

of life' of such a precious and versatile

component, it is better to take a few

looks and perform more checks!

16

for battery capacity – and not as a measure

for resistance, performance, safety or

so. But trust us, this is complex enough.

Phenomenology of degradation

SoH is widely used in degradation contexts,

i.e. how much is left after so-andso-many

years, and this is not wrong, but

certainly also not the whole truth. The

fact alone that the amount of dis/chargeable

energy depends on the conditions,

and that disbalance between cells have a

significant impact on the system performance,

suggests that not only long-term

In the most general and

widely accepted definition,

SoH is a momentarily

available capacity,

measured in a balanced

(!) battery by discharging

(!) from BMS 100% SoC

to BMS 0% SoC, using a

constant current (or power),

and that result divided

by a reference value,

e.g. a nominal type-plate

value.

effects play a role here.

In the most general and widely accepted

definition, SoH is a momentarily available

capacity, measured in a balanced

(!) battery by discharging (!) from BMS

100% SoC to BMS 0% SoC, using a constant

current (or power), and that result

divided by a reference value, e.g. a nominal

type-plate value. You might recognize

that this sentence – and we even simplified

it – offers ample room for interpretation,

argumentation and dispute. Words

like 'momentarily', 'available', charging

vs. discharging, constant current vs. con-

The capacity that you experience

in daily operation,

and during capacity

tests, is first-and-foremost

software-defined;

it is not necessarily

affected by degradation,

depending on the implemented

BMS strategy.

stant power, 'capacity', and 'reference'

allow different perspectives. Let’s start

with the most stunning: 'available'.

It might not come as a surprise that the

BMS has a word to say about how it

controls the battery and e.g. the voltage

limits. We learnt in recent articles that it

maintains an 'operational' SoC, the one

we see on our displays and tools, and that

0/100% does not 'technically' mean empty/full

– it’s just what it makes available

to you, retaining some technical reserves.

When degradation stays hidden...

So far, so obvious. But imagine the BMS

always making the same amount available

to you, year over year? You bought

an asset with 300 kWh capacity, and you

perform a test at delivery: Voilà, 300

kWh. And you use it heavily, and in year

5 your test yields: 300 kWh. Some might

be proud (“Look how careful and battery

stress-aware we are!”), some might be

skeptical (“How can that even be, why

does it not degrade?”).

This little thought experiment is very relevant

and various battery manufacturers

employ this 'Eat the buffer' logic: excess

capacity is installed, and constantly 'eaten

away' by degradation – but shielded

This graph compares technical capacity

(red) and operational capacity (blue) over

time. It shows how the BMS gradually reduces

the buffer to maintain stable performance

and mask early degradation.

from the user as long as possible. Until

the technically available capacity has degraded

so far that it falls below the programmed...

Please note: the capacity that you experience

in daily operation, and during

capacity tests, is first-and-foremost software-defined;

it is not necessarily affected

by degradation, depending on the implemented

BMS strategy.

You need proof? Here you find both the

technical (reddish) and the operational

(net) capacity (blueish) for a ca. 500 kWh

e-bus for 1 year, analyzed by volytica’s

continuous monitoring engine. You will

clearly observe at least two things:

1. The BMS of this e-bus employs such

'eat the buffer' strategy: the operational

capacity stays absolutely constant…

2. …while the 'real' capacity does, albeit

slowly, show a downward trend.

In ca. seven years from now, the buffer

will have been consumed, and the BMS

will need to reduce the operational capacity

in line with the rate of degradation.

Without such advanced and continuous

analysis, owners and operators are left

with very labor-intensive manual capacity

checkups, which never assess the technical

capacity, but only the – here always

constant – operational one.

Keep the buffer

Of course, there is another such philosophy,

lets call it 'Keep the buffer', which

is also frequently employed and feels

more natural to most people – because

it, at least apparently, does degrade: the

operational capacity is not kept constant

by software, but it decreases more or less

parallel to the technically available capacity

– requiring much more sophistication

Continuous monitoring data from a 500

kWh e-bus over one year, showing stable

operational capacity while technical capacity

gradually declines, reducing the internal

buffer.

from the BMS, as it must have means to

closely track the actual, electrochemical

capacity, much more frequent and precise

as in the other case. If it can’t, because it

is technologically a challenging endeavor,

and it assumes a rate of decrease that is

steeper than the actual degradation, the

perceived loss of performance and value

is unnecessarily high!

SoH, end of life and safety

A hard-to-eradicate rumor has it that

a battery is at its 'end of life' once the

Hands-on tips

Ensure you develop a clear understanding

of the different interpretations of

State of Health (SoH) and maintain a

continuous, holistic view of residual capacity

across your fleet. Manual capacity

tests alone cannot provide these

insights.

Be cautious when interpreting manual

capacity test results, especially those

provided by OEMs. The additional

cost of continuous, advanced battery

capacity analysis (including technical

capacity evaluation) is negligible compared

to the operational and financial

impact it can deliver.

SoH reaches an OEM-defined threshold,

say 70%. There is already a kind-of obvious

catch here, namely that 'the' SOH

is the operational, day-to-day available

net capacity, which is programmed and

decided by the BMS. As we learned, a

battery might have plenty of technical

reserves left, even if the BMS-released

SoH already reached said threshold. We

have seen cases of operational SoHs that

reach 70.0% exactly on the last day of an

8-year warranty, but our analysis yielded

plenty of remaining technical capacity

reserves – evil to him who evil thinks.

But even if speaking of 'end of life' is

grossly misleading – it might be the end

of the warranty, but for sure a battery is

not dangerous or in other forms 'dead'.

Safety for instance is, if at all, only weakly

linked to residual capacity! Many other

factors are way more important and significant

to judge the end of life!

17

THE

BATTERY

CYCLE

#6

STRESS LEVEL:

WHAT HURTS?

Battery aging is driven less by time and cycles than by

stress. How temperature, SoC and charging behavior

combine determines degradation speed—and small

operational changes can dramatically improve longevity

The previous article was dedicated

to (one prominent) effect of

degradation, the fade of capacity.

Everything in the business evolves

around that aspect – but what we did not

touch: what does actually hurt? What influences

the rate of degradation and which

measures can be taken to decrease the progression

of capacity fade?

Each battery cell type has

their unique susceptibility

to usage and external influences.

Some cell types

are tailored to withstand

low temperatures, others

high currents etc., and as

most often in life there is

no such thing as a free

lunch. An optimization in

one aspect comes at a

price in another region –

more on that later.

What are 'capacity' and SoH?

As we learnt in the last article, the capacity

that one 'experiences' in daily operations is

often referred to as the 'operational' or 'net'

capacity of a battery, and it is programmatically

made available by the BMS software.

As such, it is not 'per se' automatically

proportional to the technically available,

theoretical electrochemical capacity of a

battery system. The former might even be

kept constant ('Eat the buffer' philosophy:

see previous article!) over years, while the

latter, the technical capacity, does degrade,

albeit unbeknownst to the user.

The fact that the BMS might or might

not release the full potential of a battery

– there are good reasons for not doing it!

– shall never mislead an owner or operator

to not treat and operate a battery in a

suboptimal, high-stress way, however!

Then, which are the factors that directly

influence degradation, and thus at least indirectly

also the capacity and performance

available in every day’s life?

The short answer is: there are only a few

universal rules. Generally speaking, each

battery cell type has their unique susceptibility

to usage and external influences.

Some cell types are tailored to withstand

low temperatures, others high currents

etc., and as most often in life there is no

such thing as a free lunch. An optimization

in one aspect comes at a price in another

region – more on that later.

Calendar aging vs. cycle aging

First of all, let’s introduce the concepts of

calendric (or calendar) aging and cyclic

aging. While not used, i.e. no current is

flowing in/out, a battery is degrading solely

calendric. And as soon as current is

flowing, this is overlayed by cyclic aging.

Both modes lead to capacity fade (and resistance

increase) by ions falling irreversibly

dead by becoming trapped in side

reactions, those reactions obviously being

influenced by different factors. Therefore,

we call these factors influencing factors.

Typically, the ions react irreversibly

with other materials in the cell (such as

the electrolyte), forming passive residue*.

There are also other forms of residue that

can become dangerous.

The influencing factors are quite universal,

only their individual impact differs

strongly:

- Temperature – i.e. cell temperature and

not the temperature of the environment**

- SoC – During idle periods (calendric

aging), aging depends on the SoC at

which the battery is stored. During opera-

tion (cyclic aging), aging depends on the

SoC window, i.e. the charge range between

the upper and lower cycling limits.

- Current (only during cycling aging) –

the charging and discharging current

Often mechanical stress like vibration and

pressure is (correctly) included into the

list; for the moment we will ignore them.

Temperature and battery lifespan

How do these factors influence the rate

of capacity fade? It depends on the cell

chemistry’s susceptibility, and this dependency

is often highly nonlinear. Most

cell chemistries appreciate being stored a

low temperature, slowing down calendric

aging (lowering the probability of side reactions).

However, particularly NMC cells on the

other hand suffer heavily from being

charged at low temperatures (<5-15°C),

especially at high C-rates*** – which of

At a glance

1. Each subtype of Li ion cells has their

unique and often complex susceptibility

to external influencing factors – and

calendric aging (parking) must be distinguished

from cyclic aging (operation).

2. Both result in different forms of Li

ions becoming inactive – always leading

to a more or less pronounced fade of

capacity (and increase of resistance),

and sometimes even leading to dangerous

residue building up (Li Plating).

3. Different usage conditions thus lead

to different stress levels (or degradation

evolutions), and just counting cycles

or age can never result in an adequate

assessment of degradation!

4. In other words: one must still check

the capacity and SoH regularly and independently.

5. This also dictates an elevated awareness

for both dangers (potential induction

of safety issues, see inlay and

following articles) and potentials of the

impact of different influencing factors

on the rate of degradation – e.g. by

monitoring the battery stresslevel.

6. Slight adaptations to usage profiles

can reduce battery stress significantly

and help to extend asset lifetime by 10

to 50%, and in some extreme cases

even by the factor of 2 or more!

course is a conflicting requirement!

One example**** of a high-quality NMC

cell: the slowest rate of degradation here

is at 15°C, slightly better than at 25°C. But

going another 10°C colder, to 5°C, literally

destroys the cell.

Lithium plating, thermal runaway...

Li-Ion batteries can be regarded safe. 'Self

ignition' can, if at all, only be assumed if

internal microscopic defects accumulated

to critical levels. In simple terms Li-ions

travel from one pole to another inside a

cell, while electrons take that journey

through the electric circuit of the vehicle.

Under normal conditions, the ions migrate

into the opposite electrode to be reunited

with the well-travelled electrons. Yet if it

is cold, the current is high (fast-charging)

and/or that host electrode is already wellfilled,

this migration becomes as cumbersome

as boarding an already packed

plane with another three dozen shivering

passengers. So, under such conditions, it

is electrochemically often more favorable

to form metal deposits on the electrode –

Lithium Plating, the 'bad form' of residue.

A continuous repetition of this process

stacks up such defects, potentially up to

the point of penetrating filaments that

cause internal short circuits (dendrites).

Those in turn may generate heat, which is

vital for speeding up chemical processes,

leading to even more heat and gases being

generated – a self-sustaining process called

'Thermal Runaway'. Such a situation

becomes ballistic in less than five minutes,

eventually violently venting gases in

the form of electrolyte-soaked, thick white

vapor clouds.

Note: the conditions that facilitate plating

are only partially controlled by a vehicle’s

battery management system. It is the

Li-ions travel from one pole

to another inside a cell,

while electrons take that

journey through the electric

circuit of the vehicle. If it is

too cold, the current is high

and/or that host electrode

is already well-filled, this

migration becomes as

cumbersome as boarding

an already packed plane

with another three dozen

shivering passengers.

So, it is electrochemically

often more favorable to

form metal deposits on the

electrode – Lithium Plating,

the 'bad form' of residue.

A continuous repetition

of this process stacks up

such defects, potentially

up to the point of penetrating

filaments that cause

internal short circuits (dendrites).

Those in turn may

generate heat, which is vital

for speeding up chemical

processes, leading to even

more heat and gases being

generated – a self-sustaining

process called 'Thermal

Runaway'.

operator’s responsibility to monitor and

foster their batteries, just like careful storage,

handling and sensible monitoring of

flammable liquids is unquestioned bestpractice.

SoC comes into play

So while C-rate (current) and temperature

Hands-on tips

1. Plan for passive balancing. After

charging to 100 %, follow the OEM's

recommendations for idle time to allow

the BMS to balance the cells effectively.

2. Request granular battery diagnostic

data. Ask for SoC spread, cell voltages,

and deviation metrics, ideally at cell level,

or at least per module or string.

3. Specify it in tenders. Require a defined

balancing strategy, a max cell imbalance,

and long-term access to raw

battery data.

19

THE

BATTERY

CYCLE

typically form a complex interplay, and

susceptibility to temperature often differs

from calendric to cyclic aging, the real

surprises are brought to us by: the SoC!

The SoC can have a significant impact on

battery lifetime, both the idle SoC during

parking (calendric aging) and the SoC

window or range in which an asset is operated

(cyclic aging). While again only one

of hundreds of examples, it is quite telling

which potential lies hidden in that factor:

in the following figure, the degradation

evolution of two almost identical use cases

of a high-quality NMC cell is shown.

Both use only c. 50% of the battery (not

uncommon), and the first (green) does so

in the upper half (100>50>100>…%, i.e.

returning half full to the depot), while the

second one (purple) has a daring driver:

starting at 50% and returning virtually

empty (50>0%). The impact on degradation

is incredible: the second battery will

live 2-3 times longer than the first one,

reducing total cost of ownership by more

than 50%.

Another – quite extreme – example of the

same NMC cell: Both cases have the same

energy throughput (i.e. result in the same

vehicle range); yet a daring driver that

doesn’t operate in the 'safe zone' (100>50%

SOC), but dares to reach the depot at c.

0% in the evening (starting at ~50%), will

extend battery life by c. 2-3x!

Who dares wins! Really? When it comes

to these susceptibilities, generalizations

are very dangerous. There might be ample

amounts of cells that react completely differently

– one often quoted rule of thumb

is that by avoiding high (>90%) and low

(<10%) SoC regions, lifetime can be greatly

extended. However not in this example

– it’s not depicted, but an operation 'in

the middle' (75>25>75>…%) even exhibits

a slightly faster degradation than the

blue one!

20

The global trend is towards

ever higher energy

densities (longer range),

which is often dearly paid

for by sacrificing lifetime.

Yes, average stability is

going down! This is particularly

true for passenger

car automotive cells, but

might, sooner or later, also

affect heavy duty vehicles.

On battery stress level

That sounds complex, confusing and almost

unmanageable. And yes, not even a

trained expert can assess how stressful a

given usage profile is even by analyzing

it manually, i.e. by assessing C-rate profile,

temperature evolution and SoC ranges

and comparing them to the multi-dimensional

susceptibility matrix of the cell

type*****.

To reduce complexity there is the concept

of the 'stress level', a simple number that

tells the observer whether a given use

case is more or less stressful than a nominal

reference case (such as the default

design and warranty conditions like 'End

of life after 8 years') – a stress level of

1.0 means that a given case is circa as

stressful as planned, and a stress level of

2.0 means that the battery under the given

influencing factors is probably degrading

c. two times as fast as planned.

There are tools that can automatically

analyze stress level in a monitoring platform.

These tools are useful for assessing

risks and potential, as well as adapting

use cases.

The role of BMS

There is a discussion ongoing – for years

now – of whether or not it’s the battery

electronics’ (BMS’) task to control the

influencing factors and keep them under

control. Yes, it is. It will prevent detrimental

usage that might cause imminent

threats for safety ('very high stress'). But

except for very rare instances, it is neither

designed nor intended to make recommendations

for the ‘most optimal’ stress level.

In other words: I will prevent you from

slurping two bottles of red wine, but it

will not recommend drinking one cup of

ginger tea instead.

Interestingly, the global trend is towards

ever higher energy densities (longer range),

which is often dearly paid for by sacrificing

lifetime. Yes, average stability

is going down! This is particularly true

for passenger car automotive cells, but

might, sooner or later, also affect heavy

duty vehicles. In any case, the severe and

profound susceptibility of all Li ion cell

types to the influencing factors should

alert us all and raise the awareness for the

huge optimization potential that might lie

dormant in so many use cases.

Doubling or tripling charging power leads

to more overpotentials ('foam'), which

reduces energy intake compared to a

base-scenario: tripling power can result in

less than doubling effective energy intake!

[Source***]

* Often referred to as „solid-electrolyte

interface“(SEI), as this residue forms an

interface between the solid electrodes and

the electrolyte.

** The cell temperature is of course never

entirely independent of the ambient temperature.

Especially in battery systems

without temperature control (no active

heating/cooling), cell temperature is directly

influenced by the ambience. And

even if a system is actively controlled, the

cell temperature will approach ambient

temperature after prolonged off periods.

*** 'C rate' is the normalized current; instead

of speaking of 20A – which can be

a lot for a small (e.g. 2Ah cell), but negligible

for a large 1000 Ah battery – one

often divides the current by the nominal

capacity: 10C in the first case, 0.02C in

the latter. 1C thus means: empty in c. 1h.

**** Charging and discharging @1C, full

SoC window (0 – 100% – 0%), temperature

varied

***** This susceptibility matrix is often

called stress map. And it is most often

even unknown, at least in parts, to both

cell and system manufacturer because of

the sheer extend and complexity. And as it

allows for the said great optimization potentials,

it is also often a well-kept secret.

THE

BATTERY

CYCLE

#7

DATA TRANSPARENCY

AND OWNERSHIP

Battery data is abundant, but value depends on trust and

access. Who can use vehicle data, how KPIs like SoH are

defined, and how results are interpreted ultimately shape

operational decisions, warranties and asset value.

Data is the new oil - people keep

saying. But like with loads of

crude, thick, stinky and gooey

oil, how to distil value out of it

is the real question. This article shall put a

general light on the question of data, ownership,

usage rights, and more specifically,

the trustability of data and what can be

extracted from it.

When we speak about data, we generally

mean every information that comes

from, or is associated with, an asset like

an electric bus or an electric truck. This

entails meta information like data sheets

and technical parameters that are (almost)

not changing over time, like type-plate

capacity, used battery cell type, the manufacturer

of the doors, etc.

Data, data and more data...

But the real amounts come from time series

data, that is, sensor readings collected

during operation of the vehicles. There

are more straightforward signals like the

passenger compartment temperature, the

number of passengers, or whether the

doors are open or not.

But if we keep focusing on the battery

alone one realizes how much data these

systems actually handle every minute:

every cell in a battery pack has one voltage

sensor that is read at least 10 times

per second. Similar with current and temperature

sensors. So a battery produces

thousands sensor readings per minute,

and gigabytes per month. Thanks to many

initiatives like the VDV 238 quasi-standard,

today more often than not this 'raw

oil' is accessible by attaching the right

hardware to the communications interfaces

of buses, like the FMS. Often, this in-

A battery produces thousands

sensor readings per

minute, and gigabytes per

month. Thanks to many

initiatives like the VDV

238 quasi-standard, today

more often than not this

'raw oil' is accessible by

attaching the right hardware

to the communications

interfaces of buses,

like the FMS. Often, this

information is also transmitted

to a Cloud.

21

THE

BATTERY

CYCLE

At a glance

1. Battery data is plentiful, but insight

depends on structure, resolution,

and trustability, not on data volume

alone.

2. State of Health is not a physical

fact but an interpretation that strongly

depends on available data, assumptions,

and processing methods.

3. Regulations such as the EU Data

Act improve formal access rights,

but real-world data access and usability

remain primarily a contractual

and technical issue.

4. Manual capacity tests and single

SOH values provide only a limited and

often optimistic view of real-world

battery performance.

4. Continuous, independent analysis

of operational battery data enables

a more realistic understanding of residual

capacity, degradation behavior,

and asset value.

formation is also transmitted to a Cloud.

And an increasing number of professional

public transport operators (PTOs) and

fleet operators are starting to unlock this

treasure trove of information by employing

tools to automatically translate crude

oil, ehm, data into key performance indicators

like SoH (make sure you read

the article on this one!), safety indicators

or other crucial information to extract

the maximum from the batteries and use

these black boxes to their true potential.

Data ownership is not a joke

But in doing so, the big question arises

22

There is no such things a

'data ownership' (in the EU).

Data is legally not 'owned',

in the sense of: only one

party can do something

with it. Rather, on data

there are usage rights. So

the question should rather

be: who has the right to

use which kind of data,

and subsequently, who is

allowed to access this information

in the first place?

frequently: who is actually owning the

raw data? And if someone else than the

OEM extracts KPIs, are they better or

more trustable or 'legally usable'? What

if my own SoH assessment differs from

the OEM’s? What if I can’t get hold of

the necessary raw data to make such independent

own assessment in the first

place?

Let’s look at ownership. The sad news

is—there is no such things a 'data ownership'

(in the EU). Data is legally not

'owned', in the sense of: only one party

can do something with it. Rather, on data

there are usage rights.

So the question should rather be: who

has the right to use which kind of data,

and subsequently, who is allowed to access

this information in the first place?

Until recently, this was all but clear, but

the EU Data Act from 2025 has certainly

set new standards here, effectively giving

the owners a prominent (a) right to access,

and (b) right to use and also share

vehicle data with third parties (no such

legal framework exists in the US, to be

fair)!

Case closed, everyone can access

everything, by law? As always, there

is plenty room for debate how to interpret

and enforce this. Particularly as not

everyone would want to go into a legal

escalation over this important, but – at

least as many OEMs argue – not vital

question.

So irrespective of the abstract legal terms

and 'Acts': In the business and professional

context we are all operating, access

and usage rights to data are more

a negotiation and buying power topic,

rather than something to appeal towards

the OEMs e.g. after a purchase order is

placed. In other words: there are ample

examples of smart transport and logistics

operators that successfully strong-armed

manufacturers into granting sufficient

access to the relevant data, without appealing

or relying on law alone.

Who is right and trustable?

One question remains: if I have data,

and do some math (or have others do

the math, i.e. tools and service providers

to interpret and analyze the copious

amounts of data with electrochemical

data models), are the results better and

the OEM will accept them? Why do results

of e.g. SoH assessments differ in

When it comes to resell

value, things are not

too different. Instead

of PTO discussing with

OEM, two PTOs sit on

the negotiation table.

And again, transparency

builds trust, and this

trust enables an educated

discussion on a fair

market price.

the first place?

There is too little standardization on

how which KPI is actually defined, and

this leaves everybody plenty of room

to interpretation, and the OEMs are often

defining crucial, warranty-related

KPIs like 'SoH' themselves in the warranty

contracts. All too often however,

even this definition is missing or sparse;

needless to say that it crucial to have a

zero-ambiguity mutual understanding of

key indicators that build the foundation

of e.g. warranty contracts.

The main power of independent data

analysis is to make a level playing field.

Just because there are two capable teams

on the field now doesn’t automatically

mean that one wins. You still have to

Hands-on - tips

1. EU Data Act is nice, but still it is the

buyer to enforce data access, unless you

have a trigger-happy, well-financed legal

department.

2. Data quality is crucial; even if you believe

you have access to it in the vehicles,

make sure it is not just 'there', but in the

right resolution, frequency and quality

(e.g. check against VDV238 standard).

3. Also take care that you employ a

telematics unit that doesn’t spoil the

nice raw data during Cloud-transmission

e.g. by applying heavy & lossy compression

(what budget telematic providers

often do)

4. Don’t view a third-party / OEM-independent

assessment of any performance

indicator (e.g. SoH) as the 'silver

bullet', yet as the enablement of an educated

discussion and a level playing field.

Results will differ!

play (that is: to argue

your case towards the

OEM), but now one is

equipped with a trustable,

transparently

comprehensible and

relatable assessment

of the situation. Something

that is absent

if one must trust the

OEMs self-assessment

defenselessly. The

consequences of differing

KPI definitions

become very tangible

when it comes to capacity testing.

Manual capacity vs real-world SoH

To make it concrete, lets take a look at

manual capacity checks for SoH assessment

(again make sure you have read the

article on SoH!).

A typical procedure is to drain the battery,

for instance by driving in circles and/

or full-throttle heating, then letting it balance

for a few hours to one day – super

crucial to get the full capacity out, as we

learnt recently! – and then perform a charging

under controlled conditions, until the

battery is, or reports, 'full'.

Quite an effort, but is it worth it? Let's

look at another prototypical example: you

see only the operational SoH of a LFP bus

for 1.5 months, late last year, estimated

from telematics battery data. Typical values

of 70-80% in daily operation are visible,

quite low for a 1-2 year old bus, but

with single points close to 90%, too. The

reason for the spread is mainly disbalance

and the usage profile dependency, but the

point to make here is another: 2 OEM-cu-

Volytica’s capacity analysis of an LFP bus over 1.5 months

shows ~77% SoH in daily operation versus ~96% in OEMcurated

capacity tests, revealing a 20% discrepancy.

rated capacity tests in the fashion outlined

above were conducted in early October,

yielding two times 96% -- almost 20% higher

than the average before. Mind you,

both OEMs (not shown) and our analysis

(shown) do yield 96%.

Contractually and legally, the 96% are the

mark: according to the controlled tests

under optimal, non-operational conditions,

the battery is almost new and far

away from the (here) 70% warranty threshold.

But under operational conditions

and in real life, the operational capacity

made available to the owner is significantly

lower, and the results of capacity

tests should never be used e.g. for route

planning or dispatching without thorough

scrutiny and deeper understanding!

Manual capacity tests might yield very

optimistic results that are not per-se representative

for daily operations, and the

outcomes can only be used for operational

dispatching with a huge grain of salt.

Why KPIs differ, and how to handle it

Everything has advantages and disadvantages,

and every PTO

should take a deliberate

decision on how

to handle data sources,

data analysis and how

to stand one’s ground.

When it comes to resell

value, things are

not too different. Instead

of PTO discussing

with OEM, two PTOs

sit on the negotiation

table. And again, transparency

builds trust,

and this trust enables an

educated discussion on

a fair market price.

THE

BATTERY

CYCLE

SPECIAL FEATURE BY

Editor in chief

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Managing editor

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Layout & graphics

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Editorial management

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Printing

Industrie Grafiche RGM srl,

Rozzano (Mi)

Sustainable Bus

Milano City Court Authorization

n. 109 – September 5th 2023 National

Press Register n. 4596 – April 20th 1994

n. R.O.C. 2880 30-11-2001

POWERTRAIN International

Suplement POWERTRAIN Diesel


n. 860 – December 18th 1987 National


Poste Italiane Inc. – Mail subscription

D.L. 353/2003 (mod. in L. 27/02/2004 n°

46) Art. 1, subsection 1, LO/MI

Sustainable Truck&Van

Supplement Vado e Torno


n. 6041 – September 20th 1962 National


Poste Italiane Inc. – Mail subscription

D.L. 353/2003 (mod. in L. 27/02/2004 n°

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