Simulation and comparison of HEV battery control for best fuel economy and longer battery life

Almost all HEV battery control strategies keep the battery state of charge (SOC) within a lower limit (SOCmin) (these strategies also called charge sustaining strategies). The goal from sustaining the SOC in this way is to prolong the battery life. But the question is; what is the optimal value of (SOCmin) for a battery, to achieve best fuel economy and longer battery life at the same time?; knowing that when (SOCmin) is too low (around SOCmin=0.2) we get good fuel economy per one speed cycle but the battery dies soon and cannot perform a lot of cycles, but when (SOCmin) is high (around SOCmin=0.8) the battery can survive for a larger number of speed cycle but with poor fuel economy per cycle. The objective of this paper is to propose a method to investigate and solve this problem by simulation using Simulink environment; we used the manufacture's data of a Ni-MH battery, empiric equations, and appropriate control strategy to find the optimal value of (SOCmin). The study shows that, for best fuel economy per one cycle; the (SOCmin) value must be as small as possible, for longer battery life; the (SOCmin) value is about (SOCmin=0.85) and for the optimal case (which is the total improvement brought by the battery from first time use until its end of life); the optimal (SOCmin) value is about (SOCmin=0.7).