Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyThermal characterization and modelling ofLithium-based batteries at low ambient temperatureDomonkos Szente-Varga, Gyula Horváth, Márta Rencz{szvdom | horvath | rencz}@eet.bme.huAbstract – In this paper our recent results on batterymodelling are presented. In the presented work Li-Po batterieshave been examined. The methodology of measuring batteriesis discussed in details. The measurement setup is shown onblock diagram level. The paper demonstrates how themeasured results are evaluated, and how a mathematical modelcan be created, which is ready to use for simulations andprediction algorithm developments for devices supplied fromLi-Po batteries.I. INTRODUCTIONNowadays energy issues are getting more and moreimportant. Energy is supplied mostly by batteries for mobiledevices. The lifetime of the battery depends not only on thepower consumption of the devices. Energy efficiencyenhancement is a very important issue in such sensornetworks and sensor nodes that have to tolerate extremeambient temperatures (e.g. meteorology stations and spaceapplications). The active controlling of the battery’stemperature is in most cases not, or only partly possible, butone of the greatest effect on the cells capacity and lifetime isthe temperature. In this case, it is very important to haveextensive knowledge about the operation of the batteries attemperatures lower than 0° C.To efficiently work with batteries, lifetime estimations,prediction-algorithms, energy-aware methodologies areneeded, and these requires models. The characterization ofthe batteries used in the areas depicted above is a greatchallenge. It is important to know the amount of charge thathas been taken from the battery and its behavior in a verylarge temperature range is also needed to be understood. Forthat very reason we developed a new measure arrangementthat allows for the exploration of this problem.The energy efficiency of the Lithium-based batteries canbe determined at low temperatures with the help of ourmethodology. Using the obtained models the energy balanceof applications can be calculated. Furthermore the energybalance will be optimized for semi-active batterytemperature controlling methods.Differing from the previous works, a new type of batterieswas examined from the viewpoint of the temperature andload dependence. The experiences we had earlier collectedby Ni-MH batteries, supports us to expand our cognition byLithium-based batteries.II.METHODOLOGY OF MEASURING BATTERIESAll that we know about a rechargeable battery is its outputvoltage and the temperature. We can extend this knowledgewith the state of the charge in the battery. For example firstwe can fully charge the battery, and monitoring the battery’sstate is very important, because this way are able to tell howmuch and how the energy was drawn out from the cell. Thisis very useful information, because there is a great emphasison the methodology of discharging the battery, as the cellbehaves differently due to the rate capacity effect [3].Another important property of the rechargeable batteries isthe recovery effect: after a greater load leaving the cell inidle (load is switched off) the output voltage starts to risewith the time. This is due to the diffusion of materialsstoring the energy inside the cell. Behind the rise of thevoltage stands real recovery, further amount of energyreleases which lets the users more charge to pump out formthe cell [2].III.MEASUREMENT SETUPThe setup has to be able to measure Li-Ion and Li-Pobatteries at low ambient temperature. For this purpose wehave chosen a thermal test chamber with streaming air andregulated temperature. We have placed fully chargedbatteries into the chamber, and then fully discharged them.This means, that the charging of the batteries was not carriedout at low temperature, it was done in room temperature.The discharging current was always constant, i.e.independently of the voltage of the cell, we forced the samecurrent from the battery during a discharge cycle.Measurements could be constant current experiments, but wealso performed periodically varying (pulsing) and impulsetype load discharge measurements.The measurements were made with an equipmentspecially designed for Li-Ion battery dischargemeasurements. This was developed at our department [ref.].It consists of two main parts: an analogue and a digital part.The cell joins to the analogue part with four wires. The fourwires are important for the accurate measurement of thecell's output voltage, the big charging/discharging currentflows on two of the wires through the power supply's currentgenerators and the battery cell, and on the other two we©EDA Publishing/THERMINIC 2008 128ISBN: 978-2-35500-008-9