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Fig. 6Cell voltages measured in a 3,000 Ah battery afterfour years' operation without cell-voltage equalisers,and six weeks after equalisers havebeen installed. The cells in the diagram are arrangedaccording to the values recorded in thefirst measurement• Initial reading• Reading after 6 weekstery voltage was measured every fifteenminutes; the results are shown in Fig. 7.As is evident from the power drawn fromthe battery - 104 % of the nominal eighthourcapacity - it had been restored to perfectcondition.In a battery without cell-voltage equalisers,the balance between the cells is restoredthrough periodic equalising charges.Since the battery backup time in practiceis determined by that battery cell which isin the worst state of charge, the battery willnot - in the intervals between the additionalcharges - meet the backup-time requirementscalculated on the basis of nominalbattery data. Correct dimensioning, therefore,requires that a battery without cellvoltageequalisers should have somewhatgreater capacity than a battery providedwith such equalisers. It is difficult to quantifythe need for overcapacity: it dependsvery much on the maintenance of batteriesat each specific plant. But a plausibleestimate is that the cost price of batterieswith cell-voltage equalisers is roughly thesame as that of batteries without equalisers.The two alternatives - greater batterycapacity or automatic control - are in allprobability equally good. On the otherhand, cell-voltage equalisers make thebattery more resistant to insufficient maintenanceand in this way help to assure thatit is in good condition when its capacity isneeded.This means that battery maintenance canbe made easier. Experience has shownthat the number of voltage readings can behalved, and substantial reductions of costshave been demonstrated in many cases.Relatively large cost savings can also bemade when new batteries are installed andwhen battery cells have to be exchanged.When a new battery is installed, it is normallycycled three or four times, in orderfor all its cells to be in the same condition.In this procedure, which takes three to fourdays, the battery is measured each hour.It has turned out, though, that a battery providedwith cell-voltage equalisers seldomrequires this cycling and measurementprocedure. Such a battery is in perfect conditionafter six weeks of normal float-charging.When the equipment to be powered isinstalled and tested, the battery is well balancedand in good condition.If the capacity of a cell in an installed batteryhas been considerably reduced, dueto premature ageing or some kind of dam-Fig. 7Capacity test of the battery shown in Fig. 4, threemonths after installation of cell-voltage equalisersERICSSON REVIEW No. 1-2, 1992