AudiOn19s

Gents,
I know there are several threads on this but what I'm seeing is somewhat weird and hoping for a little insight.

''04 E55 with 75k on the clock

I got the battery warning message after the car sat recently for a few weeks. It triggers immediately after start up. I do not get the message pertaining to accessory functions unavailable like it got last year when my accessory battery was bad.

Primary battery is original to the car
Accessory battery was replaced with an OEM unit last year
Voltage regulator is original to car
Voltage at primary battery with car off is 12.4
Voltage at primary battery with car running is 14.0
Voltage at accessory battery with car off is 11.8
Voltage at accessory battery with car on is 11.8

The no change in voltage at the accessory battery is the weird one to me. Getting 14.0 at the primary battery when running rules out voltage regulator right?
My gut tells me the Original primary battery from 2004 is the issue but the fact that all of the voltages check out fine there has me confused.

desired_speeds

Are you getting the red battery message? The accessory battery is definitely not supposed to be 11.8V. If it's still running the original battery, then I recommend replacing it first. If you continue to experience the same issue, then it may be your BCM not allowing the battery to get charged.

AudiOn19s

Yes red message. The constant 11.8 to the accessory battery is what has me scratching my head regardless of running or not

In reading quickly on the BCM I thought it only controlled when items are shutoff I've not found anything that says it controls charging current to either battery.

Hulk

The aux battery is not constantly being charged, the relay under the cowl sends charge to it when it needs it.

Rocman8

+1. Not many know about this.

Papieye

A battery in good condition should put out around 12.7V 24h after you drove it.

das eighty

So the suggested course of action is replacing the relay underneath the cowl?

jmb614

I would probably have the Aux battery tested before replacing the relay. I'm on my third aux battery now.

Hulk

Ya replace the aux battery first

Yacht Master

So the battery warrning comes on, telling you that there is a problem with your battery (that is the 9 year old orginal battery) and you come here to ask what to do?

How long do you think a battery lasts?

You can't check anything untill you have a good main battery installed.

FeelThePower

I had the same message appear after I left the inside light on overnight and drained the battery. The next morning I jump started the car and the battery visit workshop message was on for the next week.

Both batteries were amongst everything else was fine.

Ended up removing just the negative terminal on the main battery for about five minutes. No battery message after that. You will have to reset the pano. roof afterwards.

I would try this first before throwing money at it. Next step of this fails would be taking the car to a shop with STAR and get them to read out the fault codes for you.

Good luck

moosejaw

Check one of your SAMs
Same thing happened to me which left me stumped for several months. Only was to check is via STAR

I replaced both batteries, aux relay, and voltage regulator before I discovered it was my drivers side SAM. The part cost me $118 at mb

jmb614

Happen to have a part number and actual part name for the SAM moose?

Rocman8

Couldn't the reason you've had 3 aux batteries be because maybe your relay is bad? If your relay is bad or going bad that's a good reason the aux battery keeps failing.

Style_Front

I just changed out my accessory battery since it wasn't being charged. How do I know if the issue lies with the relay or the SAM

tw2

As above. You can also pull codes for BCM too.

Style_Front

What does BCM stand for?

tw2

Battery control module. Sits under the dash if I remember correctly, left side of my car (RHD).

Obaid1999

Hey guys I've been searching all threads about red battery warning lights here for weeks and see a lot of different cases with different causes leading to the same issue. And since the charging system comes with so many controllers, that's what got me lost. Sometimes it seems to me that I knew what the problem was since I had done some diagnostics on the system, but when I intend to replace what I think is the cause of the problem, I hesitate and get confused for fear of replacing the wrong part . Pardon the length of the introduction. In general, before discovering this issue, I had done something on the same day that may have been a mistake and the cause of the problem, and it may not be, but before I say what I did to it, I will tell you about the state of the charging system before, in order to avoid the possibilities of wrong diagnosis. Exactly a year ago I had a battery drain issue, so I assumed the batteries were bad, since they were around 5 years old,so I put new batteries from mercedes dealer but it didn't solve the issue, next I replaced the voltage regulator but did not solve it aither. after that I had to go harder with the diagnosis, at the end it was the front passenger seat control module, replaced it, and everything becomes fine, After that I didn't drive it for six months as I went to another state for work and now I'm back. Of course the battery was disconnected, and here I will mention the thing I did that made me suspect that it could be the cause of the problem, and without it I would have suspected on the alternator only. since I don't know how old it is. Ok what happened is my trunk can only open when the system is powered, so It can not be open with the key blade due to bad trunk lock, so I had to connect my battery charger at 2 amps settings to the alternator. the red clamp to the B+ terminal connector at the back of the alternator and the black clamp to a ground point. I got the trunk opened and recharged the batteries and Guess what?!! I forgot to reconnect the auxiliary battery terminals and started the car. A few minute later, All Electrical consumers started to turn off one after another. I remembered that I forgot to reconnect the auxiliary battery so I reconnect it back and started the vehicle again، But nothing changed for the better, and after about ten minutes the red battery sign appears. That's when I realized that the car running off the battery not the alternator, so I checked the voltage on the alternator with the car on and off and it was the same voltage as of the battery, so the alternator doesn't produce any power, because the voltage doesn't increase when I start the car. now the question is, Is there a possibility that what i did caused the system to short out? I'm suspecting a shortage either on the auxiliary battery relay or the ×4/10 fuse. i know I've said a lot but hey it is a nightmare I am sure that everyone who has suffered from this problem feels me, and I hope everyone who has experience and can help does not skimp on me. any help will be appreciated, thanks