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Cell-voltage EqualisersSeries BMP 160Johan FrandforsIn telecom installations, batteries are used to guarantee uninterrupted service in theevent of a power outage. The dimensioning of battery capacity is based on estimatesof the frequency and duration of outages and other disturbances. The condition of thebatteries is also an essential parameter. Inadequate charge of individual battery cellswill reduce the time electronic equipment can be powered by batteries. In 1982,Ericsson Components AB introduced cell-voltage equalisers as a means of securingfull charge of all cells in a battery.The author describes Ericsson Components' new cell-voltage equalisersBMP 160 003, BMP 160 004 and BMP 160 005 and how they can be employed tosecure proper charging and increase battery service life.consequence, that these cells will be discharged.Other cells are supplied with acurrent in excess of their need and areovercharged. To improve the condition ofthe first-mentioned cells, the battery mustbe subjected to equalising charge at regularintervals, with a current that is capableof recharging those cells which havenot been compensated for their self-discharge.Other cells will inevitably be overcharged,which has a negative effect ontheir service life.equaliserscells (electric)power supplies to apparatusThe principle of cell-voltageequalisersA 48 V battery contains 23 or 24 cells connectedin series. Each cell is subjected toa self-discharge process, the intensity ofwhich depends on manufacturing tolerancesand the battery's age. Float-chargingof the battery, in normal operation, is oneway to compensate for this self-discharge.If no cell-voltage equalisers are used, allbattery cells are supplied with the samefloating current, which means that for someof the cells the floating current is lowerthan the self-discharge current and, as aThe cell-voltage equaliser is a current/voltageregulator (Fig. 2), which is connectedacross each single battery cell. The rectifierthat feeds the battery is set at an outputvoltage equal to the nominal cell floatingvoltage multiplied by the number ofseries-connected cells in the chain. If allthe cells were identical, all of them wouldhave the nominal voltage and, as appearsfrom Fig. 3, an additional current of 200mA would pass through the shunt regulatorin each cell-voltage equaliser. If a cellhas a voltage below the nominal value, thecell-voltage equaliser will reduce the currentthrough the shunt regulator, and thecorresponding higher current will passthrough the cell. If the cell voltage exceedsthe nominal value, the equaliser will increasethe current through the shunt reg-Rg. 1Cell-voltage equaliser BMP 160 005/1, connectedto a battery cellERICSSON REVIEW No. 1-2, 1992
29JOHAN FRANDFORSEricsson Components ABulator and so reduce the current throughthe cell. In this way, each individual cell willhave a current whose value is determinedby the deviation of the cell voltage from thenominal value. This means that, in a fullycharged battery, each individual cell will besupplied with the floating current neededto compensate for its self-discharge.Cell-voltage equaliser BMP 160Fig. 1 shows a BMP 160 cell-voltage equaliserconnected to a battery cell. The equaliserweighs 40 g and its dimensions are30x23x9 (HxWxD). It is connected directlyto the cell poles by means of one red andone blue 270 mm cable provided with ablade-type contact. BMP 160 comes inthree mechanically identical designs whichare in different colours: BMP 160 003/1 for2.25 V is red, BMP 160 004/1 for 2.23 V isblue, and BMP 160 005/1 for 6.75 V isblack. The equalisers for 2.25 V and 2.23 Vare intended for individual cells: the onefor 6.75 V is intended for three-cell batteries.The design is simple and rugged. Theregulating capacity, 0-350 mA, is sufficientfor batteries with a nominal floating currentof up to 200 mA. If greater capacity is needed,several equalisers can be connectedin parallel across the cell.Technical data of the three versions aregiven in Box 1.Fig. 2The battery floating current is regulated in eachindividual cell and is not affected by the conditionof other cellsFig. 3Current l s through shunt regulator for cell-voltageequaliser BMP 160BMP 160 004, 2.23 VBMP 160 003, 2.25 VBMP 160 005, 6.75 VERICSSON REVIEW No. 1-2, 1992
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Page 32 and 33: 30Box1CELL-VOLTAGE EQUALISER BMP 16
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