the present invention is an apparatus and a method for determining the state of charge of a rechargeable battery . it comprises a means for charging the batter , current sensors , heat detectors for detecting the heat emanating from the battery during charging , with the heat being the complement of the charge acceptance percentage , and a processor for storing a mathematical charge acceptance model for providing the relationship between the charge acceptance percentage and the state of charge of the battery , and for calculating the state of charge of the battery from the heat detected by the heat detectors and the charge acceptance model . an accurate measure of battery charge state is produced while the battery is being charged . this provides information concerning the operability of the battery and also allows efficient charging to avoid energy waste . the present invention is adaptable to existing battery power systems without the need for significant additional components and is applicable to any system utilizing rechargeable batteries .