A system and method of monitoring the amount of charge consumed from a battery powering a device, e.g., an implantable medical device, operable in at least first and second current drain states are disclosed. In each drain state, a current-sensing resistance correlated to the drain state is connected with the battery to develop a respective current drain state voltage that is converted to a current drain state count established over the duration of the drain state. The first and second current drain state counts are accumulated as cumulative first and second current drain state counts that are indicative of the amount of charge consumed from the battery. A battery charge state may be estimated from the cumulative first current drain state count and the first current state resistance and the cumulative second current drain state count and second current state resistance.