jbrinkley

New to me 2017 550, trying to figure out if I should have my tender cable on the big battery in the front or the small one in the rear. Front is the starting battery.

I've on'y had one other mercedes, that one had the big battery in the rear and small one up front. E63 2009, That one I had the tender on the big one in the rear.

Does anyone know which should be used for a battery tender on the cls? I'm just seeing strange things with the set up on the bigger front battery. Like there's some kind of interrupt at a certain time frame or voltage.

I'll test the rear acc battery in a few days assuming not many people on here and not much traffic.

ntwillie1

I would imagine the front battery. The rear one on the cls is an aux battery. At least on my 2014 CLS 550 it is. Good Luck.

konigstiger

Engine compartment see attachment.

rumb

The Eco Start battery is the small one in the trunk. I cant imagine that the first start is from the main battery and all the rest from the other one?

Tbbt CLS

Use the tender on the underhood battery. Have you had that battery load tested yet? You might just be having a failing battery causing your problems. Can you give us more detail about the problems you're experiencing? Marv

jbrinkley

I never received any notifications there were replies. That's weird.

Anyway. I'm not having problems with the batteries, I just always install one in every car.

I saw lower voltage on the big one in front around 12.5vdc. But no first crank of the day lag. Rear small was always 12.95vdc. I was thinking rear was all acc and shut down supply until it went full quiet after locking the car and going to sleep after a certain time.
I never got volts to read the same at any time.
I'm letting some months go by and see what happens, there will weeks when it doesn't get driven so I'm sure it will let me know all by itself which one does the heavy lifting.
There are way too many things I don't know about this car yet. Big learning curve.

Thanks for the replies.

Poomobile

I replaced my trunk one and my auto start stop feature started to work again (which I hate). I wonder what would happen if I just disconnected it...

jbrinkley

from reading the big m278 engine pdf I'm pretty convinced, certain really, that any battery tender needs to be on the main front battery.
*but I'm still trying to figure out the real world voltage and charging info on this set up.