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Fig. 3. Circuit fo r complete conditioner/c harger.

cell. Al l of the se problems

mentioned will p revent t he

battery from reaching the

se nsor cutoff, so that highrate

charg ing can continue,

eventually damaging the

battery.

At t hi s point. let m e

d igress. Som e experimenters

and suppliers of chargers

believe that the cha rge

sho uld be term inated aft er

replacing 130% t01 50% of

the battery ca pacity. This is

to prevent lo sing elect rolyte

within the cells. O thers

prefer the battery to remain

on a trickl e charge wh ich

may be as low as .01C of the

battery capac ity. Ce ne ra l

Electric Co m pany's Nickel

Cadm ium Battery Handbook

recommends that

nicad batteries be c ha rged

at a fairly high rate, but that

at completion of the normal

c ha rging period they be

kept on a .j Ccra te topping

cha rge . As all cells in the

battery may no t h a ve

identical c haracteris tics,

the topping c ha rge will perm

it the weaker cells to get a

fu ll charge without harming

the othe r cells . C .E . goes on

to say that mo st nlcads may

be left on the .1C cha rge

rate for extended periods of

time without harm.

Another item worth men-

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An other adva n tage

ga ined by this cond itioning

is more o perati ng time per

charge. In Fig. 1, the re lay

used should operat e on a

low current and ha ve a coi l

rating somewha t lower than

the batte ry voltage. Re sistor

R is chosen so that the

total current dra in on the

battery, includ ing the pilot

light and rel ay, sho uld total

the Ampere-ho ur rating of

your battery.

To describe t he operation

o f the condit ioner:

When a battery whi ch is no t

fully discharged and has

greater tha n 1 volt per cel l

is connected, current will

flow through t he ze ner

diode. This will place a positive

bias on t he base of the

transistor, causing it to canc

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T e ,

boning here is the memory

effect o f nicads, but since

we are all probably aware

o f this cond itio n by now, no

disc ussion should be necessary.

Evolution of the

Condi tioner/Charger

Ve ry few chargers described

in magazine articles

o r that are available from

the HT manufacturers take

into considerat ion the state

of charge remaining in the

nicad battery. If the battery

is no t depleted when put into

t he charger, this can lead

towa rd d e ve lopin g t he

memory effect just mentioned.

To prevent thi s from

happening, the cond itioner

shown in Fig. 1 was developed.

This can be built as a

stand-alone unit or may be

inc orpora ted into a complete

charger system. Before

the battery is pl aced

on cha rge, it is put into the

conditioner. It will immediately

go into disc harge at

the 1C rate. When the battery

voltage d ro ps to the

t-volt-per-cel l cutoff point

the relay will drop o ut,

thereby termina ting a ny

fu rt her d ischarge . This procedure

will no t harm the

battery and it will erase any

memory effects.

Parts list lor Fig. 1

0 1- Zener diode (1 V per cells in battery)

02-Silicon diode

Rl-1ooo Ohms, % Wall

R2-75 Ohms, 10 Walls

AY-Coil rated at less than battery voltage

01-Motorola HEP 50038

Lamp is type 387 in sui table holder.

Parts List IOf Fig. 2

IC-HEP C4058P programmable timer

Q'-HEP SOO38 transistor

D-Sllicon diode

RY-12·volt relay with dc coil resistance greater than 250 Ohm s

R1-27oo Ohms, V. W

R2-6200 Ohms, V. W

R3-1 megohm, V. W

R4-10k, 'I. W

S1-Momentary contact push-button switch

C-2.2 uF, 35 V

Parts List lor Fig. 3

T- Stancor P 6469, 25.2 ve t A

F-1-A fuse

nt-c-zener diode (1 V per celts in battery)

02, 3-Sillcon diodes

D4-1-A bridge rectifier

R1-1ooo Ohms, '12 W

R2-75 Ohms, 10 W

R3-1ooo Ohms, V~ W

R4-25O Ohms, 10 W

R5-430 Ohms, '!J W

R6-620 Ohms, V~ W

RY1 - 4POT tz-vou coil (Allied Control TF 154C-C) (P and B KHU

1701 1)

AY2-SPST 12-..,011 coil (greater than 250 OhmS)

S1 - OPST toggle switch

S2-Momentary contact push-button-type switch

L1 , 2. 3-Type 387 lamps in appropriate holders

Cl - 10 uF, 25 V

C2-4_7 uF, 35 V

C3 - .47 uF, 35 V

C4-50 uF, 50 V

01 , 2- Motorola HEP SOO38

IC1- 7812 regulator

duct. The relay is pulled in

and the discharge cycle

starts . When the battery

vo ltage drops to the t -voltper-eel!

level, the zener

diode sto ps co nd uc ting,

cutting o ff the transistor.

This causes the re lay to

d rop o ut. terminating a ny

fur ther di sch arge .

The next considera tion in

the design of the charger

was the method to be used.

I wanted a constant current

to cha rge at a fai rly high

rate . As it is necessary to

replace about 130 % of the

battery capacity for a full

charge, I decided to cha rge

at the .sC rate for approx i­

mately two ho urs to rep

lace 100% o f the capacity.

At the end of this time

the charge r sho uld switc h

73 Magazine· Aprll ,1981 107

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