M~Factor

I rarely use my 2015 ML. There was a stretch of 2 months that I did not even start it. After I finally did, I got the "Auxiliary Battery Malfunction" message. I thought this would go away after driving for a while. Not so.


Is this the main battery or one of those tiny "backup" battery that I need to replace? Is there a reset procedure just to see if this really need replacing?


Thanks in advance.

NickW

From what I've read of other's with this issue, it'll have to be replaced. Maybe take it out and try it on a trickle charge to see if it just needs recharging or it has a bad cell(s). I do remember reading that one person got there's replaced under warranty b/c their aux battery was "shorted".

pcy

Depends on how much time you got on your hands :-)

You can remove the battery and trickle-charge it and see if the problem goes away. If the problem doesn't go away afterwards, you may need to replace it.

Since it is tucked behind the main battery (at least on my truck, it is), and it is not that easily accessible, if I were you, I'd replace it with a new one and be done with it.

Diabolis

The aux battery on every W166 I've ever seen is located behind the right side rear panel in the cargo compartment (below the spare near the tail light). It's a SLA unit so I certainly would NOT attempt to trickle charge it, and at ~$50 it makes a lot more sense to just replace it with a new one.

Koenig1

Hmm... not sure, but in my 2015 ml350bt the aux?? battery in underneath the right passenger rear seat. Just flip seat base and remove cover.... or is this the main battery? I may be wrong, having not any need to search, but I believe the main battery is located in the engine bay compartment, buried in the right(passenger) side near shock?

M~Factor

I haven't look in the rear panel area but I did open the floor panel under the rear passenger seat and there is, what looks to be a fuse box there.

Diabolis

No - that's the main battery. There is no battery in the engine bay. See https://mbworld.org/forums/new-m-cla...ml#post7061119.

Diabolis

Main battery is under the front passenger seat. There's a pretty good video at .

Koenig1

You're right... fuse box under rear passenger seat. I was trying to remember why I thought that, and it was when I hooked up my trailer lights +power for the trailer module! sorry, good luck

NickW

Not sure why you would say that, there's nothing wrong with charging a SLA battery.

Diabolis

They are rechargeable batteries so obviously there's nothing wrong with recharging them - for as long as it's done with a charger that has very good voltage regulation, which is not the case with 99% of all automotive battery chargers (trickle or otherwise). Poor voltage regulation results in overcharging and thus excessive formation of hydrogen gas, which - unlike in a wet cell battery which is always vented - in a SLA battery has nowhere to go and results in internal battery damage.

pcy

^ +1.

M~Factor

I had a chance to look this weekend. I saw what looks to be a very small battery (the size of a small motorcycle battery) peeking out on the right corner of the trunk compartment. No where near the spare tire though.


If this is the one, how the hell do you get this out? lol 95% of it is under the carpet. Do I need to gut the trunk out to get to remove it?

Diabolis

Yes, that's the one. You need to take out the side panel. There are a couple of plastic "hooks" with a Torx screw in the middle - you need to take those out and then you can remove the panel.

Brent S

You can remove/re-install the small aux battery without cutting the carpet, and without removing the entire passenger side trunk liner (panel).
I just did it.
Wasn’t easy, but it’s possible.
I didn’t want to remove side trunk side panel, because I didn’t want to pull off the trunk’s rubber seal.