These batteries have very poor power density and are very inefficient. The advantages of nuclear-powered batteries are:
- They generate power over a long time, decades,
- They generate some heat.
They don’t generate much power. If you have a building, you would definitely think of a nuclear RTG as a “very shitty battery”, and that’s even if you don’t care at all about radioactivity.
Thinking of these as a “battery” is also a bit misleading, IMO. These are really just small power plants, which generate heat and turn the heat into electricity. The heat is powered by radioactive decay of Pu-238, and then turned into electricity with the extremely inefficient Seebeck effect. If you had a source of heat you wanted to turn into electricity, it’s much more efficient to use that heat to turn a turbine which is connected to a generator. And if you want an efficient, cost-effictive turbine, you make it big. At that point, you have a power plant.