2005 E320 CDI 155,000 miles

I'm getting the battery / visit workshop warning now and then.

Both batteries are 2-3 years old.

At rest, with the motor off, both batteries are reading 12.5 - 12.8. When the car is running the main battery is reading 14.4 while the aux battery is only reading 12.8.

Is this typical, Does the smaller battery have a lower charge rate?

Could the regulator be going?

Matt

 

The auxiliary battery is only connected to the main charging system in two instances:
-the first, the relay connects the auxiliary battery to the charging system for approximately 10-15 minutes per drive cycle, then disconnects the auxiliary battery from the system. If you were to measure the battery while connected to the charge system, you would see the 14.4 volts. When disconnected from the charge system, you will only see the battery voltage; in this case 12.8 volts which looks pretty good.
-In the second instance, the auxiliary battery will connect to the main electrical system upon a failure of the charging system resulting in the main battery's charge dropping unacceptably low.
At approximately 11 volts the system begins shutting down the first stage of convenience functions and at approximately 10.5 volts it begins shutting down the second stage convenience functions to save power for the SBC braking unit.

Since the problem is intermittent, and your car has 155,000 miles, I would venture it is either the voltage regulator or the alternator itself. Less likely is the battery control module. Having said all of this, failure of the electrical system will result in failure of the electronic braking system! Get it fixed ASAP!

Message in white: your car has a malfunction. get it checked out soon because it will not fix itself.
Message in red: "visit workshop", you have a bad alternator, main battery, battery control unit, or relay K57/2 failed.
Message in red: "Battery/alternator - STOP", park that puppy as soon as it is safe and get it towed home. This message means electrical system is unstable, the auxiliary battery is connected and is all that is powering your electronic brakes. When the battery goes flat, your brakes stop working except for the two tiny emergency hydraulic valves that provide unboosted hydraulic pressure to the front calipers only.

Edit: I assume unhcampus is University of New Hampshire campus. If so, I work with some of the folks up there in the Space Science area...it's a pretty campus.