Optimization of the discharge cut-off voltage in LiFePO4 battery packs

Considering that the voltage and capacity/energy level of a single battery cell cannot meet the requirements of electric vehicles, hundreds of battery cells are usually connected in series/parallel. The discharge cut-off voltage of the battery cells is an important factor affecting the consistency of the terminal voltage and the capacity usage efficiency (i.e., the ratio of the discharged capacity to the nominal capacity) for a battery pack. This paper presents an optimization method for determining the LiFePO4 battery cell discharge cut-off voltage considering three factors: capacity usage efficiency, terminal voltage dispersion, and minimum terminal voltage. By applying this optimization method to different number of series-connected battery cells, the relationship between the optimized discharge cut-off voltage and the number of battery cells is deduced, and this relationship is instructive for the operation of the battery pack. Compared with the conventional method where the discharge cut-off voltage is 2 V, the terminal voltage dispersion is greatly reduced and the maximum rate of variation is 10.7%. At the same time, the minimum terminal voltage increases by about 1.5% and the capacity usage efficiency only declines by about 0.5%. Consequently, the battery cells will not be over-discharged because the minimum terminal voltage is larger than 2 V. The rate of decline of the capacity usage efficiency is small and it can be ignored. Moreover, the level of consistency among the terminal voltage of the battery cells is improved and lifetime of the battery pack will be extended.