An Energy Efficiency Evaluation Research Based on Heat Generation Behavior of Lithium-Ion Battery

Abstract

Energy efficiency across a range of state of charge (SOC) is an important performance parameter in variable Battery Energy Storage System (BESS). A novel method was proposed to evaluate the effect of heat generation with the parameters of resistance and entropy change on the energy efficiency. The energy efficiency was evaluated through measuring the resistance and entropy change during different DOD (depth of discharge). The internal resistance was determined by constant current discharge, followed by plots of V vs I at constant SOC, and found it exhibit a reduction as SOC increasing. In addition, the entropy change was measured by the temperature change of the open-circuit voltage. Energy efficiency results were obtained at constant charge input levels and different discharge rates under the different DOD. The result show that the energy efficiency has a decreasing trend with the increase of the depth of discharge, and the energy efficiency plateaus at 60% DOD. The energy efficiency is decreased from 88.8% to 87.3% under 1C discharge from 20% to 80% DOD. Furthermore, the lower the current rate, the higher the energy efficiency under the constant DOD. We isolate the effects of undercharge and underdischarge, and find the difference of energy efficiency under different current rate increases gradually with the increase of DOD.