Abstract: A battery charge control method which relies upon the relationship between the battery state of charge and the charge efficiency. The battery charging efficiency is estimated by computing the amount of charge received by the battery as a function of input charge. Thermodynamic calculations are used to obtain an estimate of battery charge levels over a wide range of temperatures, and two stage filtering is used to obtain a useful slope from noisy charge level signals to determine charging efficiency and ultimately control the charge rate. The method is especially useful for recharging nickel-hydrogen batteries used aboard spacecraft. The battery charge in such a battery is estimated by computing the number of moles of hydrogen gas in a cell. The input charge is the integral of the charge current. Thus, the charge efficiency is proportional to the rate of change of the number of moles of gas with respect to the input charge.