Self-Discharge and Charge Retention in AB[sub 2]-Based Nickel Metal Hydride Batteries

Abstract

We studied the self-discharge and charge retention characteristics of an 85 Ah nickel metal hydride (Ni-MH) traction battery and proposed the most likely mechanisms associated with the passive capacity loss via self-discharge. The mechanistic explanation is evident through the analysis of the internal cell pressure profile and the corresponding individual electrode potential variation in the aging process. Three different stages, each associated with a predominant mechanism for self-discharge, were identified at room temperature, corresponding to a short (10-100 s), an intermediate (10 min-25 h), and a long (2 days-several months) period of aging. All three stages involved reactions with hydrogen gas, which led to the capacity loss. Most of the capacity loss was temporary and could be recovered from subsequent recharge. The effect from oxygen evolution and recombination only showed in the beginning of the self-discharge process and diminished after about 4 h of aging from the fully charged state. Through the remaining part of the intermediate stage, the residual hydrogen gas continued to reduce the positive electrode capacity via a partial discharge. In the final stage of the long aging period, the capacity loss was probably caused by a minute leakage current. From this study, we derived the charge retention as a function of aging time, which can be used for capacity prediction. Good agreement between the predicted value and the test result was found.