After 8 years of use I replaced the battery in the smart key of my 2012 ML 350 using the following steps:

1. Move the small slide on the too of the key to one side to remove the mechanical key.

2. Using the front tip of the mechanical key push in at the left of the top of the key where the mechanical key is normally located. Push in slightly and the rear black plastic cover on the key will be released

3. Remove the rear plastic cover. Note that the writing on the battery faces up. The battery number is CR 2025.

4. Tap the key body on a table and the old battery will fall out.

5. Replace the old battery with a new one with the writing on the new battery facing up or towards the back of the key body.

6. Snap the rear plastic cover back in place.

7. Reinstall the mechanical key by pushing it back in to the key body.

8. Test the smart key by depressing any of the buttons. You should see a small red light light on the key body.

The batteries are available on Amazon for about $4.35 for two. There are several U tube videos on line which show the battery replacement procedure.