Energy efficiency of a Ni-MH battery used in hybrid electric vehicles

The energy efficiency of the battery is the proportional relation between the net energy change of the battery and energy that input from the outside or output to the outside. The energy efficiency of a Ni-MH battery (40Ah) was studied by testing and theoretical analysis in order to improve the control precision of hybrid electric vehicles (HEVs) and the battery useful life. A test stand for testing the energy efficiency of the Ni-MH battery was established. The data from experiments of constant, pulse current energy efficiency performed on the Ni-MH battery were analyzed. Based on the experimental results, mathematical model of net energy of the battery was proposed by function-fitting methods, and the effects of current and state of charge (SOC) on energy efficiency were analyzed. Theoretical analysis and experimental results suggest that charge and discharge energy efficiency of Ni-MH battery are above 90% within the range of 30%-80% SOC, and the effect of current on energy efficiency is greater than that of SOC, which provide theoretical basis for selection of batteries and dynamic optimizing technology of HEV system controlling strategy.