Capacity evaluation and degradation analysis of lithium-ion battery packs for on-road electric vehicles

Accurately calculating the capacity of battery packs is of great significance to battery fault diagnosis, health evaluation, residual value assessment, and predictive maintenance in electric vehicles (EVs). However, many existing methods are not suitable for real vehicle conditions. In this paper, a capacity calculating method specialized for EVs is proposed. This method uses an open circuit voltage (OCV) correction strategy to guarantee the credibility of the battery state of charge (SOC). Its accuracy is validated by full charging carried out on 5 testing vehicles and the mean absolute error is 2.6 Ah. Data from 707 on-road electric vehicles are collected and the capacities of their battery packs are calculated through the proposed method. Taking the mileage and service life as variables, two degradation models of battery capacity are established with mean absolute errors equal to 3.138 Ah and 3.137 Ah. According to the degradation model, the battery's average service life is 71.8 months, and the average service mileage is 121,700 km. Moreover, correlations between capacity degradation and user behaviors are analyzed, and the results indicate that charging the battery with the start point at a SOC between 30 % and 40 % can effectively relieve the degradation.