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Newbie here... I have searched threads and am coming up stumped.

My new to me 2003 E320 had been sitting for 4 years when I got it the end of December. The prior owner had jumped started it many times because the battery was bad.

This car was a free one and I did not want to spend much on it until I knew what I had, turns out this thing is great but the error is driving me crazy. I bought a battery from Advance Auto (not AGM) and got it to start properly. I replaced the auxiliary battery with an AGM. I replaced the relay on the cowl. Cleaned the BCM connections, replaced the voltage regulator. I am getting 14v rear battery and 14v auxiliary when running. I bought a iCarsoft tester, I am getting a 9092 error code and it is not showing a charge going to the auxiliary battery. I also pulled off the cover to the front pre-fuse box and tested the 150 AMP fuses, they are good. Are there other fuses that need to be checked? I printed out a W211 Dual Battery System manual from Mercedes, it has been some help.

What else am I missing? Any help is appreciated.

Something is sucking ur power supply dry. I bet its an AGW related issue. Try to remove fuse of AGW and see if starts the next day.

Funny thing is the battery is not going dead, I need to check this again to verify but after I replaced the relay under the cowl the auxiliary battery was getting 14.1 volts from the alternator. To me it seems like the BCM can't talk to the front relay or battery.

I went out and tested the polyswitch fuse, it has roughly 5 ohms. With the engine off the battery in the trunk reads 12.6volts, under the hood 12.6. I started the car, 14.1 to the front battery and 14.1 to the rear battery. After the car was running for a few minutes the front battery had 12.6. In my opinion the system is functioning properly.

I am leaning towards an alternator issue even though it is charging. I replaced the regulator but it was not a Bosch, the problem was there before I replaced the regulator. Any ideas?

I have searched and tested and I am still having the problem. I removed the rear battery, sanded with a Dremel all of the terminal connections at the battery, BCM and rear prefuse block. I opened up and checked all of the rear prefuse block fuses. I visually checked the BCM wires and connectors, all pins are in the BCM. I removed the front battery and tray, I checked all of the wires under the tray and vacuumed debris from the fan intake. I put the battery on a tester and it is fully charged. I reinstalled the battery and cleaned all of the connections. I checked the relay under the cowl and connector. It did throw a code being disconnected.
I purchased an iCarsoft tester and the BCM throws a code, basically saying auxiliary battery missing or not connected. On the front SAM I got the error about CAN bus to BCM missing.
I installed an Advance Auto alternator, sort of glad I did because the original had bearing noise. Did not fix the red Battery light. I have yet to replace the BCM. Is there something I am missing besides a broken wire or faulty BCM? Someone said there was a fuse between the starter and alternator, I could not find one.

Problem is fixed! I tried a voltage regulator from Mercedes, did not fix it. I ordered a battery control module on eBay and the problem is fixed. The old BCM could not see the auxiliary battery voltage.

Nice one. Thx for posting ur findings