Mechanisms likely to accelerate self-discharge of a nickel-metal hydride (Ni-MH) battery have been investigated. A higher self-discharge rate of : sealed Ni-MH batteries compared to Ni-Cd is explained by shuttle reactions between nitrite ions and ammonia. Even if a polyolefin separator stable in the electrolyte is used, the positive electrode is an important source of nitrogen impurities. It was found that some grafted polyolefin separators have a special ability to trap ammonia. Using this specially designed separator and provided that the entire quantity of ammonia can be trapped, the self-discharge rate of Ni-MH batteries can be reduced to the self-decomposition of the charged active material in the positive electrode.