Most newer Mercedes vehicles are equipped with a Smart Key system. The Smart Key doesn't look like a key, but rather a keyless entry remote. The key uses two batteries to operate; both most be fresh for the system to function. You can obtain the proper replacement battery at any Mercedes dealer, or at many electronics stores. Mercedes recommends inserting the replacement battery with a lint-free cloth.
Turn your Smart Key over, and push down the backup key locking tab.
Slide the backup key away from the edge of the Smart Key. Insert the backup key into its storage compartment; slide the key toward the locking tab to unlatch the battery storage compartment.
Pull the battery storage compartment out when it pops up. Remove the batteries from the compartment.
Insert the new batteries. Line up the positive side (+) with the contact clamp (face up). Reinsert the battery storage compartment into the Smart Key.
Replace the backup. Slide the backup key's arm into its mount; push it down to secure it.
If you are unsure whether your battery is dead, or if there is another problem, you can test the battery using the Smart Key check lamp. Press the "Lock" or "Unlock" button on your key; look closely at the light below the panic button. If it blinks, the battery is OK; if it doesn't the battery must be replaced.