It’s a power density issue, first of all, and a security/safety issue second.

I don’t need to address the safety issue, as I’m not really qualified to be talking about Pu-238 or radiation or nuclear physics. Just suffice to say some redneck somewhere would take it apart, and there isn’t much to stop them.

The more immediate reason one cannot replace an alternator with an RTG (Radioisotope Thermoelectric Generator, which is really fun to say) is they just don’t make that much electrical power. The RTG used in Curiosity has 4 kilograms of Pu-238, and that makes a whopping 110 watts of electrical power. I have a small 4x4 vehicle with a 1.3 liter engine, and it’s alternator has an output of (I think) 55 amps. 55 amps at 12 volts is 660 watts. And that’s really not enough, if I add extra lights or a big stereo I’d probably need a bigger alternator.

The next question is, “Could one use an RTG to top up a car battery while it’s parked?” And the answer is, yes. But for that, I’d rather use a solar panel, because I don’t want people as stupid as I am to have access to multiple pounds of radioactive material.