Instead of using this on the moon, what is preventing us from putting this power source in a car and either powering the motor or constantly charging a battery?
See, some decay chains have gamma emitters in the line. Those are not good to have around people without several meters of lead, concrete, or water. All things that are a bit too heavy for a car.
Well, you'll likely not die from a source the size of a poppy seed, but you'll likely have a bad time.
Localized radiation burns on whatever body part is closest to the pellet.
If, on the other hand, you choose your fuel well, a thin sheet of metal is more than enough protection. Beta radiation can be blocked by a sheet of paper, and an alpha emitter is only dangerous if you swallow or breathe it.
This - wanted to add that it's generally not a good idea to have radioactive elements not under under lock and key as they have a nasty habit of ending up in scrap yards and the like. Most nuclear accidents are from radiotherapy and x-ray machine sources not getting disposed of correctly and getting picked up by enterprising scrappers due to the high lead content of their housing.
I was about to say no one is stopping you from dying a gruesome gamma-radiaton induced death, but I would be wrong, because I now realize you're literally shitting on the very people whose job it is to prevent that from happening.