E250 W212 auxiliary battery

[ASmithI]

Mine is a E250 CGI W212 2013.

Auxiliary Battery Malfunction. Started to get this msg but still intermittent for a few days now.

I have done this. I checked the voltages of both the starter battery and the auxiliary battery. With the engine off, the aux battery is 12.90 volt and the starter battery is 12.40 volt. With the engine at idle, the aux battery still read 12.90 volt whereas the starter battery read 13.76 volt.

My question is, how is the auxiliary battery charged? Looks like it is not being charged as usual like the starter battery.

As for issue at hand, looks like I have to change the aux battery soon.

 

[190]

At 12.40 volts the main battery is partially discharged. Perhaps in those circumstances the charging system gives preference toi the main battery before charging the auxiliary battery. I would fully charge the main battery to see if that resolves the issue.

 

[ASmithI]

I smartcharged the starter battery but the message still came up. So I finally changed the auxiliary battery with a 3rd party brand. The message has not appeared again.