My original 6 year old battery went dead while parked in the garage. Doesn't the Lord work in mysterious ways? Most likely a dead cell. Good voltage but voltage drops to nothing when hit with a load of any size. I'm learning. I searched postings for battery replacement and seeing the posts about aftermarket replacement batteries, I decided to share my experience. Sorry to say I did not try the 800-FOR-MERC delivery/installation. Having had good experience with Interstate batteries in the past, I wanted a good battery that fit like the original part. Noted in other posts, Johnson Controls makes Interstate and MANY other brands as well. Note that the W210 battery is under the back seat and vented to the outside via a small hose.
I found the exact part replacement by visiting my local Interstate Battery dealer. Although I used the Interstate web site to search for the proper part number, it returned a different part. The huge long battery that comes in the W210 is not the MTP-91 that the Interstate web site shows for the V-6. Oddly enough, it does show the long battery for the in-line 6. The correct part number is the Interstate Batteries # MTP-93 (list price of $128.95, suggested retail of $106.95-I paid $124 and did not argue since it was 5pm on Sunday) See link below. The battery has the vent in the center instead of the side case, but the MTP-93 comes with an hose kit that has a barbed adapter to join the supplied hose with the factory vent tube. I trimmed the supplied hose and inserted the barbed connector, but to insure that the connection was permanent, I used a small zip tie on each side of the barbed connector to make sure. I hope fellow W210 owners that want a full size replacement battery that fits like the original, including the hold down, found this helpful.