We won’t need to replace this diamond battery in our lifetime.
It’s just a betavoltaic cell using carbon-14. The science here isn’t really anything new outside of isolating that much carbon-14 and turning it into a diamond. Also it’s going to produce next to no power. With betavoltaics the more active the radiation source, the more power you get and the faster it depletes. Carbon-14 is radioactive but only barely. Considering even shortlived betavoltaics only produce microamps at less than 3 volts even when brand new, I’m guessing they had some really sensitive sensors in use here to even detect any power being output by the carbon-14 cells.