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33Thermal run-awayPlacing batteries in cramped spaces involvesa risk ot thermal run-away. Whenthe temperature in a battery rises, the selfdischargeprocess is intensified, which willraise the floating current. The power generationin the battery increases and causesthe temperature to rise further. Theeel Is in the centre of a battery as a ru le can -not carry off heat to the same extent as theouter cells and will therefore have a highertemperature. The cell voltage rises with increasedtemperature, which means thatthe hottest cells account for a greater portionof the total power generated in the battery.The temperature in these cells riseseven more. If a cell cannot carry off enoughheat, the process will get out of control,causing the electrolyte to boil away anddestroying the overheated cell. The plantwill no longer have any standby power.If the battery is provided with cell-voltageequalisers, even a slight voltage increaseleads to a substantial reduction of the currentthrough the cell. The feedback is negative.The risk of thermal run-away - andthus a potential source of trouble - hasbeen eliminated.Valve-regulated batteries - intheory and in practiceCell-voltage equalisers have been successfullyused in valve-regulated batteriesalso. However, some experts - basingtheir arguments on theoretical models ofthe chemical processes involved - claimthat cell-voltage equalisers should not beused in this type of battery. If low cell voltagesare caused by sulphation in the cells,the increase of the floating current achievedby means of cell-voltage equalisersclearly has a positive effect. If, on the otherhand, the low cell voltages are due tothe fact that cells have dried out, and thata recombination process is going on, thenincreased floating current will have a negativeeffect. Correspondingly, high cellvoltages may indicate that the recombinationprocess is incomplete, in which casethe floating current should be kept at a highvalue.In the operational data of valve-regulatedbatteries that Ericsson Components ABhas access to, there are no indications thatcell-voltage equalisers have any negativeeffects, even though batteries from differentmanufacturers might possibly have differentcharacteristics in this respect.SummaryIn a power supply system, the battery isthat component which has the shortest lifeand needs most maintenance. This nottoo-glamorousmaintenance also requiresthat the applicable instructions are followedin every detail.When the primary power supply fails, theproper functioning of a fully charged batterysuddenly becomes vitally important.The cell-voltage equaliser has been developedto assure that the battery is alwaysin the best possible condition and to reducethe consequences of temporary deviationsfrom stipulated maintenance routines.The costs of cell-voltage equalisers arecompensated for by cost savings. Firstly,maintenance routines can be simplified,and, secondly, a battery with cell-voltageequalisers need not be cycled when it isinstalled. In a longer perspective, the batteryeconomy will be improved thanks toideal float-charging conditions, which increasethe life of the battery cells, andthanks to the possibility of replacing damagedor prematurely aged battery cellswithout having to include any additionalmaintenance measures.References1 B.W.B Battery Systems Inc. ICE MANU­AL Rev 3.0 1991.12 Bjork, D.: Maintenance of Batteries. IN-TELEC-86.13 Andersson, H. and Bergvik, S.: SealedLead-Acid Batteries for Small Telecommunicationsplants. Ericsson Review601 (1983):4,s 222-225.4 Bjork, D.: Operation and Maintenance ofPower Supply Equipment. Ericsson Review64 (1987):1, s 9-15.ERICSSON REVIEW No. 1-2, 1992