Cambz

I have the "visit workshop" red warning light. I've determined the auxiliary battery under the hood is low 10.35V. The main battery in the rear is fine.

I've read that the system (2005 E320 W211) only charges the auxiliary battery for 10 minutes after start up. I also read that is on a relay that opens between the auxiliary battery and the main battery in the trunk once the alternator sends a certain amount of volts? Can someone verify how the system works?

I want to charge the Auxiliary battery directly with a battery charger and see if it holds a charge. Does the Auxiliary battery need to be disconnected fromthe car and/or removed to charge it OR can just leave it hooked up in the car and put the charger on it?

I'd like to put 13.8V /12Amps to it and I want to make sure I don't damage any of the system components if hookedI leave it in the car.

Also if I remove the auxiliary battery for 24 to 48 hours Will I need to reset anything on reinstall, since it's disconnected from from the main battery in the rear via relay?

Thanks
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bmwpowere36m3

I wouldn't disconnect it necessarily... just use a proper charger. The "smart" ones are pretty foolproof, like CTEK, NOCO, Battery Tender, etc... I'd keep the charge rate around 2-5 amps, as its only a 12 ah battery. More likely its simply due to replacement, but charging it and testing it always a wise move.

ot1

with the car engine off, the auxiliary battery is disconnected from the vehicle, you can leave the trickle charger on for months. Some years back my car was in storage for almost two years, I didn’t know about the aux battery or that it wasn’t being trickle charged along with the main battery. When I started up the car, I got the red warning, and took the trickle charger used on the from the main battery and I thought the aux battery was permanently done for but it came back to life after about 7 seven days, battery lasted about another year and a half. Battery Tender Plus 1.25A from Costco.