3gs charging woes

Someone want to explain this to me?
I have a cigarette lighter adapter with a USB socket on it for charging things via USB while driving. (duh).
I have an iPhone 3gs.
Had a 2g.
2g charges in car, 3gs does not.
2g charge on computer USB, 3gs also does.
same cable in all scenarios.

Does the 3g have a magic fucking sensor or something to let it know not to charge?

Same thing with my sound dock.
2g charged 3gs does not.

I don’t get it. Is the power from the CLA not upscale enough for the iPhone? Is it racist? “ewww that electricity came from an alternator, blech, i dont want it…”