Explain the 2 battery setup on the early 221's
 

A big H8 battery in the trunk and a small starting battery under hood.

1) If you put a battery maintainer on one battery, will it keep the other battery charged as well?
2) Did later models eliminate the under hood starting battery for a small under dash battery to keep memory settings alive?

yes charge either battery the other one seems happy

I guess more for emissions, handling and accountants penny pinching they ditched the large rear battery - mate with the later set up in his CL had LOTS more issues that I do ( like 4 starter motors) and more new batteries than the factory can make

the other baby battery under the dash is to try and help it come out of park if the front one dies - I guess so you can push it in a ditch where they belong

I recommend charging the rear one directly from the aux post under the hood, rather than charging the starting battery. The aux battery is the one you'd want to keep topped off.

for logistic reasons - nearest the socket and away from most people walking passed - and the fact I can totally lock the car with all doors windows and bonnet shut - for 10 years I keep them topped up ONLY via the rear battery... I have a charger to the battery positive and the negative to a body ground... its makes no difference - and is enough to keep the ignition on first position to do 45mins updating the Nav mapping so long as you can get about 5 amps going (fans and lights all off )

for programming I go via the correct post up front - but that's ignition on and pulling >30 amps

I've never seen a description either, but I do know that the largest load ever experienced in the system is running the starter motor. So, even if the front battery is closer to the engine, I'd hesitate calling it the "starter battery."

I frankly think the larger battery is in the rear as a weight distribution solution. I do know W220 had rear battery, and don't know whether it had a front one. It had the 5-speed transmission, which didn't have electric control.

The problem stems from using an electric solenoid rather than rods or cables to shift the 7-speed automatic transmission. This means should there be no system voltage--dead battery--the car cannot be moved because it can't be shifted out of park. Auxiliary battery or second battery is then required as a backup should main battery fail. Certainly with the 2009 update and the itty bitty auxiliary under the dash, that little battery isn't going to run the starter but it will activate the shifter solenoid so the car can be moved (pushed).

It seems clear the 2006-2008 configuration may have been overkill. Some of it perhaps a carryover from W220.

The front battery on a pre-facelift car IS the starter battery. That's all it does. And you're right, it's small with a big battery in the rear for weight distribution, as well.

It could be that the facelift cars had lower parasitic draw and could get away with the single battery solution (with the tiny battery for the ISM).

Hi - I have a similar problem with my CL - where is the aux post under the hood to charge the rear battery?

Many thanks,

David

there isn't one.... for twin battery cars the correct procedure is battery earth and the taller post under the cover of in pre-fuse box beside the front battery (it has a 1/2" unthreaded section for the clamp) - this connects both up and charges both batteries

Hi - thanks for your response.

Is it the far right connection on the photo below (the one with the thicker cable)?

Many thanks

David


https://cimg0.ibsrv.net/gimg/www.mbw...81a1bc8b69.jpg

can't see it in your pic - as I said one is noticeably taller with a 1/2" unthreaded section for the clamp


https://cimg5.ibsrv.net/gimg/www.mbw...3e6ade7723.jpg

don't let that positive connector touch the inner wing !!!!! - stupid stupid design

and top up that OLD front battery - might go for another few years if you get some water in there (likely the two end ones are about 1" down and showing the battery's plates

then pull it out, get the black base plate out and clean out the leaf litter before your blower motor takes a bath !!! - there's a drain under there likely blocked up

Thanks - I am off on Friday so I am going try trickle charging it then - I will let you know how I get on.

The battery in the front is always trickle charged so hopefully in a good state.

David