Mercedes-Benz vehicles come with "Smart Keys" that serve as key fobs for entry to and ignition of the vehicle. The Smart Keys are fitted with small batteries like those used in watches. Like any battery, these batteries sometimes die and need to be replaced. If your Smart Key doesn't seem to be working very well, or if it doesn't work at all, it may be time for a new battery. Instead of taking the key fob to a Mercedes dealer, save yourself some time and money by replacing the battery yourself.
Remove the mechanical key from the Smart Key by sliding the button on the end of the Smart Key and pulling the mechanical key straight out.
Insert the mechanical key into the opening on the end of the Smart Key and push it in until the cover on the back of the key opens. Remove the cover to expose the battery tray.
Tap the Smart Key against your palm until the battery falls out. Use a lint-free cloth to insert the new battery, with the positive side facing up.
Reinstall the battery cover by inserting the plastic tabs, then pushing the cover closed. Test the buttons to make sure the new battery works.