Techniques for determining whether a medical device will be able to deliver stimulation according to a particular program throughout a useable voltage range of a power source of the medical device are described. According to some examples, the medical device charges a charge pump to a level sufficient to provide a stimulation output according to a stimulation program, determines a length of time that the charge pump charges at the present power source voltage level, and determines a time between stimulation pulses of the stimulation program. Whether the medical device will be able to deliver stimulation according to the program when the power source is at a power source voltage level lower than the present voltage level is determined based on the length of time the charge pump charges at the present voltage level of the power source and the time between stimulation pulses.