Power source longevity monitor for an implantable medical device. An energy counter counts the amount of energy used by the implantable medical device. An energy converter converts the energy used into an estimate of remaining power source longevity and generating an energy longevity estimate. A voltage monitor monitors the voltage of the power source. A voltage converter converts the voltage monitored by the voltage monitor into an estimate of remaining longevity of the power source and generating a voltage longevity estimate. A calculator is operatively coupled to the energy converter and to the voltage converter and predicts the power source longevity using the energy longevity estimate early in the useful life of the power source and using the voltage longevity estimate later in the useful life of the power source.