WEBSRFR

In case the main battery gets drained for whatever reason, is the car smart enough to tap into a functioning auxiliary battery to start the car in an emergency?

If this is the case is there a connection between the auxiliary and the main batteries that might potentially drain both batteries if there is a short circuit or an issue with something continuously consuming power? Or is the auxiliary battery tapped only if the main battery is not functioning properly for emergency starting?

Thanks for any info. Just thought it would be good to know...

Diesel Benz

The AUX battery is isolated with a relay from the main battery and the relay is not closed before the engine is running. It is too small for the starter motor anyway.

WEBSRFR

So basically if the starter battery is dead, the other battery is no help for an emergency start? I guess this has been implemented purely to keep an errant light or electronic device left on from completely draining the battery?

vettdvr

1) Yup! main battery dead car won't start.

2) NOPE.. the backup battery is power for the SBC brake system to stop the car in the event of electrical failure.

SterlingG

where is the backup battery.

starbrite

Passenger side engine compartment, under the HVAC cabin filter. Near the firewall.

WEBSRFR

Anyone know or have a list of what is supported by the "main" starter battery and what is supported by the "other" battery? When command is on with the audio system on and navigation turned off, is that using the "other" battery? How about moving seats when the engine is off? Is it possible to use the instrument cluster to determine the health of both batteries?

MBE550

For those of you who have 2007-09 E550, have you actually seen your Aux battery under the hood?
I was at the shop yesterday to check my battery because I had to jump start my car after leaving my headlights on for about 3 min.
They could not find aux battery anywhere in the car. They checked under the hood, under the rear seats. I did notice there is some space where the battery supposed to be; passenger side under the hood, that's large enough to hold a small battery.

Did they remove aux battery for 2007+ W211 V8 models?

starbrite

I believe it's still in the same location. Give me an hr and I'll post a pic of where it is.

Barry45RPM

I thought in the 07+ without SBC, the Aux battery was no longer needed for power to the SBC unit, so it was eliminated.

MBE550

I just checked owner's manual for 2006 through 2009 and only 2006 manual has info on both batteries; main and aux. In the tech data section, only 2006 shows the spec for the aux battery.

It looks like it's not just my car that's missing the aux battery.
I'm amazed that this was not mentioned anywhere in the forum or any other site that I googled. Owners of 2007-09 assumed that there was an aux battery I guess.

I forgot to add that the shop I visited called MB dealer to confirm the location of aux battery for 2007 model year, and the dealer tech insisted that the aux battery is under the hood.

ALLDATAdiy INFO

Auxiliary Battery G1/7 as found in ALLDATAdiy.com

This is in the Right Rear of the engine compartment.

This is for an 08 MY

MBE550

Interesting. So this is passenger side right?
I don't have one in there, and mine is MY07.
Did you take this picture or you got it off of the website?
As I said, a lot of website out there wasn't updated for MY07 changes.

Update:

https://mbworld.org/forums/e-class-w...shop-code.html
Yes, it's eliminated since MY07.

starbrite

Post #12 shows the battery with the HVAC Cabin filter removed.

This is the cover/filter on my '04:



If you have one, battery is under that cover.To remove unscrew the circled bolt in the front.
Next, pry away, with a screwdriver, the latch at right rear.
Lastly pull up from the front and release from 3 catches at the rear.

PITA to reinstall the cover !

Barry, I don't know (and, of course there's been a lot of discussion on how many or what devices are powered by this battery) if it's SBC alone.

MBE550

Thanks for the photo.
I have that exactly same cover, but there is no battery under that cover.
I took the car to a reputable MB/BMW specialist, and they could not find one, and were surprised that there wasn't aux battery.

The shop tested my main battery and it was operating within the spec with load test.
I don't know why the battery was drained in the first place with under 3 min. of headlights/radio use while the car's not running.
Maybe the HID pulls a lot of amperage..
Between the time I had to jump start my car and to the shop, I only drove less than a mile / 5 min. and the battery was fully charged. Weird.

konigstiger

MB discontinued the following on board electrical system components as of 6/1/06 (MY07): auxiliary battery, battery control module and auxiliary battery relay replaced by battery sensor. I would suggest shop retrieve rest current data with DAS (see page 2).

WEBSRFR

Yikes! That is good to know. Here I was thinking I had an aux battery to run things like the audio system when the engine is off and I would not have to worry about draining the starter battery. I now know to be careful of using the car's electronics when the engine is off...

pinebaron

You get a warning if battery starts to run low so you know when to switch electronics off or start the engine.

WEBSRFR

Ah that's assuming someone is around to see it (if lights or something is left turned on accidentally)! I wish it cut off any electric consumers when the battery charge is too low to preserve the ability to start the engine.

konigstiger

Although no display in IC consumer groups 1 & 2 simultaneously shut down @ 10.5V (see page 1 of previous document).

Diesel Benz

Which consumers do you think would be left on to run the battery flat? Assuming you have ignition off, all interior lights would get switched off already before the battery runs out.

Exterior lights are not turned off because the car would not know if it would be a safety issue (if the car had a failure and had to be left in a dangerous place).

gnma

I often wondered why this was, and I now I know!

starbrite

All good info !!

MBE550

My car battery died again at a car wash, and had to jump start it.
I'm waiting for MB roadside to replace the battery.

I need an opinion from someone who knows about this stuff.
My battery charges very quickly after the jump and I can restart no problem. Each time this happened, it was the headlight(HID) that was on for about 1-2 min.
So I did some testing at home. The battery was at 12.6V - engine and all electrical off. Then I switched on the headlight for about 1 min. The voltage was 12.1V while the light was on. Lastly, I turned off headlight and it's still at 12.1V and the car cannot start.

Does this sound like simple battery issues. It's about 4.5 yrs old. When I had battery problems in the past, it will not be able to recharge fast and end up with subsequent failure when starting the engine. Basically the battery can be fully charged but will discharge quickly with headlight use.

MBE550

I just had the battery replaced by MB Roadside for free of labor charge.
The guy actually works for the local MB dealer.
This is good info for people who needs to change the battery.
Do not go to the dealer. It literally took 5 min. to install and 5 min to recync windows and test everything.