A method for estimating capacity of a battery in an implantable medical device includes obtaining a characteristic curve of voltage versus resistance for the battery, periodically determining voltage and resistance of the battery, and comparing the resistance with a third derivative function of the characteristic curve, the third derivative function having roots representing near depletion of the battery and end of service of the battery, respectively. Remaining time of service of the battery can also be determined by comparing the resistance value with a curve of battery capacity versus resistance for the battery.