seawind

I just bought a 05 E500, it looks like the previous owner had a aux battery replaced at autozone and they put it in backwards. The negative terminal on the aux battery was completely fried. I replaced the aux battery but unfortunately the red battery/visit workshop message is still there. The vehicle drives fine, everything works as far as I can tell which I'm assuming means the alternator is good. If I let the car sit and not drive it for a day it still starts fine the next so I'm assuming there is not a battery drain. I think I probably need to replace the battery relay but I'm no Mercedes expert. What do you guys think?

Appreciate the help! I will update when the problem is resolved.

Russell Ormerod

I have never had a case of reverse aux battery yet. But I would read the faults first. And yes Check the aux battery relay and the fuse

seawind

Unfortunately I don't have the equipment to read the faults but I think I might know a independent mechanic that could for me on Monday.

After sitting over night before starting the main battery is at 12.32 and the aux battery is at 12.36. At idle the main battery is at 14.20 and the aux battery is at 14.15.

If the main battery is detached the aux battery still powers all the lights so I'm assuming the wiring harness is fine.

I looked at the fuse chart but can't find a battery fuse. Is there one?

I think I found the aux battery relay on the passenger side if you are looking from the front of the car to the right of the air filter. It is a bit different then your typical car relay but if I'm testing it right I'm getting 0.05 ohm.

seawind

Solved, bad battery control module. My family member has a similar 03 E500 so I swapped the battery control module in the trunk and the error has disappeared! Unfortunatly I have to give theirs back lol, battery control module ordered now I just got to wait a few days for it to come in...

Russell Ormerod

You can flash the battery control module with new software. Try that first

seawind

Don't you have to have Mercedes equipment to do that? I thought about going that route but I'm assuming when the aux battery was hooked up backwards it probably fried something in the module. I found a used module for $165 which is probably less then what Mercedes would charge me to flash it.

Critter

Think about replacing the main battery also, 12.3V is low for a fully charged battery.
12.6v minimum
Your running voltage is good though so alt is good.

seawind

Is 12.3 low after sitting over night? I'll check the battery again after I replace the battery control module, I know I checked it once and got 12.6 but I had probably just turned it off during troubleshooting. Thanks for the heads up!

cetialpha5

A fully charged new battery at rest with no load is supposed to be 12.7 at 70 degrees. Voltage will be lower at lower temperatures. It's possible that it isn't fully charged. Also older batteries won't take a full charge but are still good, mine reads 11.9 right now, but then again, it's an 8 year old battery and in the winter it's as low as 11.5v. I believe the warning comes on when it hits 11 volts. Note that with the battery connected to the car, the voltage you're reading is not an open circuit no load reading.

Here's a state of charge chart:

http://modernsurvivalblog.com/altern...-charge-chart/

seawind

I think I know everything I need to know about a battery now! Thanks for all the responses everyone. The easy fix made this car a great deal.