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

Almost all HEV control strategies keep the battery state of charge (SOC) within two limits this strategy is called charge sustaining strategy, the goal is to prolong the battery life. But the question is; what are the optimal (SOC) limits for a battery, to achieve best fuel Economy and longer battery life?; thus the objective of this paper, is to use the manufacture's data of a Ni-MH battery, empiric equations, and appropriate control strategy to find the optimal SOC limits. The study shows that, for best fuel economy, the SOCmin value must be as small as possible, for longer battery life, the SOCmin value is between 0.8 and 0.9 and for best total improvement (the optimal situation), the SOCmin value is between 0.6 and 0.8.