Don’t skip the betavoltaic battery, (or the brand-name: Betacel), which turns beta-radiation directly into electricity. They used them in the 70s to power pacemakers, since batteries were kinda shit back then, and implanting Prometium into people is just too epic not to do.

Nowadays we have tritium-decay betavoltaic batteries, on satellites, buried or underwater sensors and probably some too secret military stuff.