Actually, one more problem. Radioactive decay is not something you can just turn on and off (well, apart from fission chain reactions, that is). That strontium in your hypothetical car battery would be steadily releasing energy whether you're using it or not. You'd either need to be venting it as waste heat (a LOT of waste heat, if it's powerful enough to directly drive your car), or using it to trickle-charge an on-board "real" battery - in which case you'd have rather defeated the purpose of having the strontium battery in the first place.
