State of health estimation of lithium-ion batteries based on the regional triangle

Abstract

A novel fast state of health (SOH) evaluation method for lithium-ion battery is proposed. The concepts of the regional voltage and regional triangle are introduced based on the incremental capacity analysis (ICA). The ICA method is only used to obtain the voltage value corresponding to the main peak on the IC curve. After that, the voltage value corresponding to the main peak is determined on the original charging curve and is defined as the Vpeak. And then a regional triangle is defined by the Vpeak and the two terminal voltages of a certain regional voltage interval with the Vpeak as the midpoint on the original charging curve. The two edge lengths containing the highest voltage vertex of the triangle are defined as the regional edge lengths and are used as the health factors for SOH evaluation. The SOH models at different sampling frequencies are established using the charging voltage data of nickel–cobalt–aluminum (NCA) lithium-ion batteries and lithium iron phosphate (LFP) battery modules at different current rates. The results show that the SOH has a linear positive correlation with either of the two health factors and the maximum R-square (R2) of linear fitting can reach >0.99. The proposed health factors still have high R2 with SOH modeling in the narrow regional voltage interval at sampling frequency as low as one point every 5 min whether NCA or LFP, suggesting that the regional triangle method has a potential for SOH online evaluation of batteries in engineering.