It's surely in your owner's manual.

For example
http://mb.braingears.com/124_DISC2/Program/OM/E320_E420/OM_114.pdf

"12V/62AH" is specification enough.

Probably what your parts man will call "the small one" to distinguish from battery for a diesel.

__________________
Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles