I inherited a 2006 E350 base model, and one of the messages is the common RED visit Workshop Batt. warning. After searching, a bit, I found the AUX batt dead (3v), not chargeable. It was dated 2005, so it was original. I replaced that battery, and the message didn't go away. Voltage readings go up on my main batt when the engine is running (12.5 v off to 13.8 running), but the aux battery does not see a rise when running. Due to this, I was wondering if anyone can explain how the charging system works. Is there an isolator that shifts charging between the two batteries? Is there a breaker or fuse on the aux batt that may prohibit charging? I will run a scan tonight, but if I can get some background on the system, it would help me diagnose. Search didn't provide me with an answer. Thanks.

 

Page 24 explains on when the aux battery gets charged.
Mike T.

 

Thank you! Will the error show on the display until it's reset?

 

I do not know. Voltage at the battery should be at least 14 volts when running. What is the mileage on the vehicle. The alternator brushes are a known item that wears out, but not until approximately 140k miles.
mike T.

 

It has 140K, alternator was serviced 20K ago. It's properly charging the main battery, that's why I asked this question. After changing the aux battery, the warning is still up. I was not sure if the alternator had a dual optput or a single with external switching.

 

Did you charge up the aux battery before you put it in? I seem to recall that it doesn't charge it that long. I don't know what you mean by the alternator was serviced. Was it replace or was just the voltage regulator replaced?

 

I think I found the issue

 

I take it cleaning up the corrosion fixed the problem? Hopefully some vinegar was able to clean that off. Just had the same message pop up on mine and hope to find a couple minutes this weekend to get into the aux battery compartment.

 

I had pins corrode to nothing, so I needed a new module. Also, one terminal had corrosion so I replaced just the terminal end. I sourced the terminal from a BMW dash harness

 

Was that the relay module for charging the auxiliary battery? Wouldn't be the first corroded module I've had to replace. The last one was the alarm module in the front driver side wheel well when the battery on it leaked and corroded the circuit board. They put batteries on the damnedest things.