Consumer Battery Not Charging
 

I have a '07 S600 with a draining consumer battery.
I finally went ahead and replaced both consumer and starter batteries.
Charged them both reading 12.79 Volts holding steady.
Started the car and at the starter battery voltage reads 14.49 Volts from the alternator.
At the distribution box, the consumer battery reads only 11.99 Volts and starts dropping gradually until it reads 9.3 Volts. Accessories start to shut down one after the other.
Now if I rev it to 1500 rpms for 2 seconds the voltage goes up to 13.49 Volts and stays there. The consumer battery begins to charge then at idle.!?!?
I don't know what to do next.
Do you think a fuse or relay is fried or is it the voltage distribution box acting up and not linking the charge ??
It seems you have to rev it to excite the charging of the consumer battery.
That should normally be charging at startup, correct?
Thanks to all for any help on this issue.

I remember I accidentally touched the positive cable from the consumer battery a split second to the chassis. It sparked but no fuses were blown anywhere I could see. However, inside that prefuse box are numerous fuses that are not visible to us.
Do you think one of these fuses could be blown preventing the charging of the consumer battery at idle?
When you rev the motor up a bit, the higher rated fuses work and charge the consumer battery?

Hi mate how did you solve the problem? I got the same problem on S class w221 2008 S 320 cdi. I don't know what to do I have change the bough battery front and rear.Have you solve the problem?

S550 problems
 

It could be the "Vehicle power supply control unit" (Bordnetzsteuergerät) that controls the charging of the batteries.
It is an active module that decides which battery is charged when.

Consumer battery drain, starter battery at 14.6v
 

Hi there, any answers, as I have this problem on a W221 S500 2006 model. The starter battery charges, the consumer battery drains. The alternator was replaced. Its now in diagnostics at an Indie, but they were already talking alternator.