Requires Two CR2025 Batteries.
Only a few dollars on Amazon Link: http://amzn.to/2doSewC

Learn how to replace your smart key fob battery. It's very simple. If your key looks different, see instructions with pictures for all keys

http://www.mercedesmedic.com/keybatt...

Get instructions with pictures as well.

How to replace your car key remote fob battery on Mercedes Benz A, C, E, S, SL, SLK, CLK, ML, vito