Deep Discharge Protection
Consider what happens during discharge:Initially, both cells are discharged in parallel, which is not a problem as long as cell B still has capacity. A problem occurs, when cell B is completely discharged, the other cell(s) still have remaining capacity and a connected load is still supplied. Further discharge of the cells with remaining capacity through the load results in a charge of cell B with reverse polarity.
Thus residual capacity of cell A can not be removed by further discharge, otherwise B cell will become permanently damaged within a very short time. A deep discharge protecting circuit disconnectig battery and load has to prevent that!