I just installed the third new AGM battery into the 1991 560SEL. After installation, the car started right up as it should. After four days, the battery was deader than a door nail. I put it on the charger and it is now holding charge. Please check out the attached photo; a friend says two of the red wires go to the alternator although I have not traced them myself. Even so, I see no reason why a brand new AGM battery would die after only four days.

Attached Thumbnails

 

First off, you have an aftermarket wire amd fusible link coming off that (with what looks like a poorly crimped terminal), so thats not stock, and looks suspiciously janky. Second, break out your multimeter. You likely have a strong draw somewhere and youll have to test circuits for current while the car is off. Open up the fuse box and go circuit by circuit to determine wjat is consuming current while the car is off. Most common offenders are stereo systems. In your case, I would start with whatever those wires are coming off the battery before tackling the fuse box.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Strange looking arrangement. I'd start by removing anything from the + battery post that was not from the factory and see if that stops draw.

__________________
Jim