Originally Posted by jay_bob

Beware on the W210 and other models with the battery within the passenger compartment:

The battery on these models is not a standard "wet" battery as found in most under-hood applications. The OE battery is designed to be leak proof in the event of a roll over. I believe this means it is a gel or AGM type battery. These batteries are more costly, but they don't require periodic addition of water and they last longer than standard batteries.

One of the reasons the under hood battery cars get battery tray rust is that the electrolyte can escape the battery under normal operation.

If you have an aftermarket, non OE battery, in a vehicle with an interior battery compartment, I would highly recommend you change back to an OE battery.

Source: Owners manual for my W210