Since the battery of the pacemaker is sealed within the generator chamber, the entire chamber is replaced when the battery is low. Most modern pacemakers use lithium batteries that need to be replaced every five to 10 years, depending on how dependent the heart is on the pacemaker.

Pacemaker batteries will not run out unexpectedly. When a battery is running low, the elective replacement indicator (ERI) is activated. Physicians can detect this activation during a routine office visit. In addition, the battery status and the general functioning of the pacemaker can be tested over the telephone. This is generally done every one to two months. Pacemakers will continue to function for approximately six months after the ERI is activated, allowing ample time to schedule an elective replacement procedure.