Aux Battery Dead
 

Hi - I have been away for a few months and tried to open my car 2009 CL 500 without success.

I had a trickle charger on the battery under the bonnet, thinking this would keep the car ticking over, however, I have since found out from reading the forums that there is another auxiliary battery at the rear.

I am unable to access the interior of the car because there isn't enough power to operate the central locking, luckily I left the bonnet on the latch so I can access this and I can also access the boot compartment.

Before I remove the rear battery to charge, I was wondering if there is an easier way to charge it - as I have read somewhere that there is a battery terminal under the bonnet, unfortunately, they didn't share the location.

Any help with this would be greatly appreciated.

Thanks

David

If the main battery in the bonnet won't take or hold a charge, you need a new one. Changing it should make your locks operate.

I have had the car rescue out and they did a battery test and confirmed it was fit and healthy - the battery under the bonnet is around a year old and always had a trickle charger on it, so I am assuming it is ok.

The car only does around 2-3000 miles a year so doesn't get much use.

Are the locks definitely operated by the front battery?

Thanks,

David

I thought the main battery powers everything, and the aux battery is there to allow access and release the parking brake in the event the main battery goes flat. Any chance you can get a battery jump from a friend?

I thought the battery at the front powered everything, but after reading on different forums and sites I am not sure now.

Without taking the panel out that is behind the seat, you don't seem to be able to access the terminals to attach the leads to and that is why I was asking the question as to whether there was an easier way to do this?

Thanks,

David

There's another forum thread on this at https://mbworld.org/forums/cl55-amg-...2009-w216.html
Is there anything there that helps? Looks as if the aux battery might do everything but run the starter.

Thanks - I saw that too.
I also found a thread that shows you how to trickle change the aux from under the hood, so I am going to give that a go tonight and I will let you how I get on.

At the risk of being redundant, up and to the left of the starting battery under the hood is a long skinny plastic cover with 4 squeeze clips on it. Lift that off and you will see a (brass or copper? post with no threads. That is the hot for the battery in the trunk.
Connect your charger to that and use the start battery ground post for the negative.

Don't know what charger you have, but the battery in the trunk is (I think) an AGM, so set accordingly.

I don't know if there is a difference between CL600's and CL500's, but you get the idea.

Anytime you park these "electronic marvels", remember they will suck the life out of the "toy" battery quick like a bunny, so always keep a maintenance charger on it. I keep one on the start and a separate one on the "toy" battery.
I use NOCO genius 5's.

Good luck and let us know.
Mike

Hi - thanks for your message - I think the wire you are referring to is the thickest wire in the picture (the one at the bottom), is that correct?

Do you just then connect the -ve to the negative on the other battery?

Many thanks

David

https://cimg7.ibsrv.net/gimg/www.mbw...7e844dc271.jpg

Is your car right hand drive? It looks backwards to my CL600. I would guess the bottom one with the big wire on it is your positive post for the rear battery and yes, use the start battery ground. Merc's share a common ground. It's the positives that are distinct.
Let me know how it goes.
Mike

I'll post a photo later of the place to charge the rear battery under the hood. You are in the wrong spot in the photo. In my 2008, there is no aux battery, despite some info online to the contrary. You can take out your key blade and use it to open the door. I believe the process is turning it twice to the left.

Here you go. My pen is pointing to the positive for the battery in the trunk. Attach negative to nearby ground.
https://cimg6.ibsrv.net/gimg/www.mbw...63f0e40405.jpg

nath_h: That's it exactly!

Thanks for the excellent backup!

Hope this helps the OP.

Thanks for your help and clear photo - I will have a look this evening and try and find this connection to attach my trickle charger to it.

I will let you know how I get on.

Best,

David

This depends on year of car.

With 2009 facelift (W221), there is only one battery ... under the hood.

On those cars, there is a super small auxiliary battery under the dash over the driver's feet that exists solely to get the transmission out of park should the main battery fail.

Yes, the visible facelift for C216 occurred 2011 but W221 (S class) facelift was 2009 and both cars have the same electrical system. My 2010 C216 has the single front battery and under dash auxiliary battery electrical system.

So, when there's a battery question, post the year of car.

Note: I've tried to tell the moderator ... there is no such thing as a W216. W means 4-door sedan, C means coupe. SLs are R ... roadster

Charging both batteries
 

Good afternoon friends! I'm going to try to clarify a little the charging of both batteries (trunk and starter) with this image. (CL550 c216 2007). I hope it works. greetings!

https://cimg3.ibsrv.net/gimg/www.mbw...c474dd4f51.jpg