Charge performance of a commercial nickel metal hydride traction battery system

We have investigated the charge performance of a commercially available 85Ah nickel metal hydride (Ni-MH) battery system. Each of the Ni-MH battery modules contains 11 prismatic cells designed for electrical vehicle traction applications. This work focused on understanding the operating mechanism of the charging process and the influence of different constant current charge regimes, with rates ranging from C/8 to 1C, on the charge performance. We found that the cell voltage profile was dominated by the potential change of the nickel electrode in the charge regime. At C/4 (20 A), which is commonly used in the charge algorithm, the modules can be effectively and fully recharged with a low degree of overcharge of 1-2%. When being charged at C/2 (40 A) with a proper control of the cell pressure (at a pressure lid of 6.8 atm or 100 psi), the modules can be recharged with a high efficiency to 93% state-of-charge (SOC). In this case, we found that the oxygen evolution accelerated after reaching 70% SOC, and an "O-2-oxidizing recombination" mechanism was present, based on the observation of the potential variation of the metal hydride electrode. When the modules were charged at 1 C rate, we found that, in addition to the oxygen evolution/recombination cycle, another "H-2-reducing recombination" mechanism was present and was related to the occurrence of "-DeltaV" in the final stage of the charge regime. The presence of the -DeltaV resulted in a noticeable capacity loss at high charge rates. Interestingly, most of the capacity loss can be recovered after several recovering cycles operating at a very low rate (e.g., C/8). We attribute this temporary loss of reversible capacity (rechargeability) to severe hydrogen gassing during the high rate charge at the metal hydride electrode and the associated "reversed oxidation" via the H-2-reducing recombination on the nickel electrode. This phenomenon subsequently resulted in a significant temperature rise in the cell, causing localized dryout, and a capacity mismatch between the two electrodes. (C) 2001 The Electrochemical Society.