Multi-step constant-current charging method for an electric vehicle nickel/metal hydride battery with high-energy efficiency and long cycle life

This study sought to investigate methods of charging nickel/metal hydride (Ni/MH) batteries for use in an electric vehicle(EV). The specific conditions for the multi-step constant-current charging method with regulation of voltage and d T/d t were varied in an attempt to improve high-energy efficiency, shorten charging time, and increase cycle life. A commercial battery system with 12 modules was subjected to a discharge/charge cycling test using patterns of dynamic stress test (DST) 120. Two-step charging with a first-step current of 0.5 CA (1 CA=95 A) was completed in less than 2.5 h after DST120-pattern discharging to 80% DOD. Further, three-step charging with the first step of 1.0 CA reduced charging time to about 1.5 h. Multi-step constant-current charging provided a high-energy efficiency of more than 80%. The battery system withstood over 1800 cycles in a cycling test with reduction DST120-pattern discharge of 20%, but reductions in constant-current discharge of only 7%, due to a gradual decrease of discharge power, independently of the value of the charging current.