Mercedes: Mercedes wont start, independent mechanic, petroleum jelly

Question
Drove the car on WednesdayFriday went to start car, got the usual lights and a message, could turn ignition, but car wouldn't start.  Tried again a little later, and got nothing at all, couldn't even turn the key.  Charged the battery Friday night.  waited a few minutes, was able to start it again.  Saturday afternoon, was able to unlock the door with the remote key, but key wouldn't turn in ignition.  then when I tried to lock it remotely, it wouldn't lock.  Will replacing the battery take care of these problems?  I believe the battery that is currently in the car was replaced in 2007.  The car is a 2001 C240.

Answer
Loss of electrical will cause the symptoms you describe. I would take the vehicle to an independent mechanic or auto-parts store and have them check the battery and the charging system.
The battery probably should be replaced,particularly if you live in a cold climate.
I would change the battery in the remote key as well.

Sometimes the computer and or the security alarm gets confused and requires resetting. Some security systems kill the ignition.
Another component of these issues is corrosion of fuses, and ground straps. I would also clean the battery post and terminals, use the anti-corrosion felts under the terminals or petroleum jelly on the post to help keep away corrosion. Clean and grounding straps, and I would also remove the fuse if they are the ceramic bullet type. Polish the points with fine emory cloth or sand paper.

Good Luck
Rob