Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. Such implantable devices are typically powered by a battery. When the battery's useful life has expired, the implanted device is typically explanted and replaced. Therefore, it is often useful to know how much battery energy has been used and/or how much battery energy remains.