The R230 and W211 both use two batteries, but for different reasons.

As previously posted, the R230 uses the little battery under the hood only to crank the engine. The big battery in the trunk does everything else.

The W211 uses the little battery for back-up power for the SBC brakes. I know, both cars have SBC brakes, so why would one model have a back-up battery for SBC and the other one doesn't??? Ya' got me! I'm sure the MB engineers had a good reason, though.