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30Box1CELL-VOLTAGE EQUALISER BMP 160 - TECHNICAL DATADimensionsWeightConnectionConnecting cableMax voltage without destructionTemperature range30x23x9 mm40 g6 mm blade270 mm2.9 V0 to +55°C(1.2"x0.9"x0.4")(1.4 oz)(32 to 131F)ModelTrimming levelColourShunt current attrimming levelBMP 160 004/12.23 +0.007Blue200 mABMP 160 003/12.25 ±0.007Red200 mABMP 160 005/16.75 ±0.015Black150 mAFig. 4, below leftCell voltages measured In a large-size batterywithout cell-voltage equalisers (ICE)Fig. 5, below rightCell voltages measured in the same battery whenit has been provided with cell-voltage equalisersfor a short periodExperiences of cell-voltageequalisersBackup timeFig. 4 shows the distribution of cell-voltagevalues measured in a large-size batterywhich has been used in a plant, underobservance of normal maintenance routinesbut without cell-voltage equalisers.Fig. 5 shows the corresponding valueswhen the battery has been provided withcell-voltage equalisers for a short period.The distribution between the cells has decreasedfrom the order of ±200 mV toabout ±20 mV; that is, by a factor of 10.This means a marked improvement of thecondition of the battery.The proper charging of a battery cell canonly be checked through several hours ofloading tests and voltage measurements.The fact that the voltage across a cell innormal float-charging is close to the nominalvoltage does not guarantee a satisfactorycondition of this cell. Nor is the oppositeself-evident: if the voltage across a cellin a battery without equaliser is up to 50mV below the nominal voltage, this doesnot necessarily mean that the cell is in abad condition. The latter, on the otherhand, is an indication that the cell mayhave a higher self-discharge than othercells in the battery and that the floating currenttherefore has not been sufficient tokeep the cell fully charged. If the worstcomes to the worst, the charging of sucha cell may be only 50-60 % of the capacityof a fully loaded one. In an eight-hourtest, the battery capacity would drop to thelower voltage limit after less than fivehours. This means a 50 % reduction of thebattery backup time in spite of the fact thatall the cells are fully capable of functioning.EconomyThe above example may seem somewhatdrastic, but low cell voltages caused by insufficientfloat-charging occasionally appeareven in relatively new batteries. Theempty circles in Fig. 6 show the cell voltagesin a 48 V, 3,000 Ah battery that hasbeen in operation for four years. The valuesfor about one third of the cells are 35m V or more below the nominal voltage. Unfortunately,this battery was not tested withrespect to capacity when the voltageswere measured, and its real condition istherefore not known. However, plant managementconsidered boost charging necessaryin order to restore the condition ofthe battery but decided to choose the alternativesolution: cell-voltage equalisers.Fig. 6 shows the voltages measured beforethe cell-voltage equalisers were installedand the corresponding values aftersix weeks. An eight-hour discharging test,in conformity with US standards, wasmade after another three months. The bat-ERICSSON REVIEW No. 1-2, 1992