A method of charging a NiMH or NiCd battery such that charging does not occupy a substantial amount of time and such that the operating temperature of the battery is reduced during the lifetime of the battery, thereby extending the life of the battery. This invention is a method of charging that is accomplished using a temperature differential method to continuously regulate the rate of charge injected into the battery. An amplifier is used to amplify the difference in temperature between one or more cases in which the battery is contained and ambient temperature. In this method, the greater the difference in temperature, the lower the charging current is injected into the battery. The method of charging may be incorporated into uninterruptible power supply equipment.