Abstract: A microprocessor samples the net effective current from a battery several times a second. A fast average current I.sub.f is calculated using an IIR filter on the net average current. A slow average current I is calculated by integrating the net average current over time with respect to time. An Ampere Hour Capacity is calculated by the use of the Peukert equation, which is split into two terms. A first capacity term is calculated using the fast average current I.sub.f and a slow capacity term is calculated using the slow average current I. The slow capacity term is then multiplied by a weighting constant C.sub.2 and summed with the fast capacity term which is multiplied by (1-C.sub.2).