The big trunk battery reads about 9.6 volts while the small underhood battery is at 12.8 volts.The two batteries aren't connected with the ignition off? The engine cranks and starts great. So that means the starter works off the underhood battery? With the engine running are they connected and being charged by the alternator together?

posts all over the forum explaining something we don't need to care about....

beside the rear battery is a module that works out what to do - if the rear battery is 9.6v throw in a skip and fit one that works

What's a skip?

He is suggesting you need a new consumer battery in your trunk since yours is substantially less than 10 volts. You should have 12.0 + volts at rest and 14+ volts when the engine is running. You are correct, the smaller battery in the nose is only for starting the engine. Unless your car has been played with, it knows which and how much to charge. Typically you get 10 decent years out of a consumer battery and maybe 4-5 on a starter battery. If your consumer battery is OEM and it was not replaced previously; you are certainly due for a new one.

The consumer battery is huge and is a glass mat battery. Probably weighs about 60 pounds. The starter battery is lead acid.

Replace your consumer battery, and keep us posted. After replacement, check your alternator output and the serpentine belt for slippage. If your alternator was the real problem, you will just kill the new battery in a short time. If the belt is frayed or the pulleys/tensioner squeaking; think about maintaining/replacing those parts. The car is incredibly reliable when it is properly maintained.

So the two batteries are isolated? With the engine on the alternator charges each separately as needed?

Yes. And yes provided the alternator output is 13.5 to 14.5 volts.

I’ll help..... Search noob!

skip /skĭp/

noun

1. A container for receiving, transporting, and dumping waste materials.

The American Heritage® Dictionary of the English Language, 5th Edition

With my 08, when the rear battery drops below 12v, weird stuff happens. If it is low, just replace it. The car will crank and run, just not properly. Interstate has a 36month free replacement. I have taken advantage of the offer!

Absolutely right. Nearly all of the car's systems run off the rear consumer battery. When its voltage drops due to age, the car tries desperately to conserve power by shutting off systems. When the voltage gets really low like under 11.0 volts, the car will do very strange things. W221 is very sensitive to low voltage.

Interstate makes very good batteries. Many other brands are actually manufactured by Interstate.

Keep us posted.

there u go the boring rubbish by the experts - still needs a battery

So where do you connect a battery maintainer so both batteries charge, short of using two maintainers, one on each battery?

either end - I do rear battery - positive to the battery, negative to a body ground (known as an earth in the UK)

officially its the front via the taller post in the pre fuse box and the battery earth

for trickle charging it makes little difference - I've used the rear for 10 years - but for programming I use the front

But if the batteries are isolated and you do the rear battery, how will the front battery get charged? And since the front battery is just for starting, why use that for programming?

I don't believe they are isolated from each other - weird electronics manage what both are up to all the time....

Xentry tells you to use the front for programming (maybe because during programming the box of trick that manages the rear might not know whats happening or maybe just because that reachable without dismantling half the car ?) - that said if I try to do via the rear for serious current from prolonged diagnostic sessions its less satisfactory, key on off cycles easily confuse the situation and it can go over voltage and when that happens it often takes a while for the power supply to wake (so you get large voltage fluctuations that don't seem to happen via the front)

OK. I've always connected the maintainer to the rear battery because I had that rear panel off. Guess it makes sense they would say to use the front for that reason.

Took me several years to figure this out. As mentioned, the front and rear batts are independent of each other, and require separate charging... Unless...

There's a plastic cover on the right wing, in front of the starter batt. Remove it and store in the skip...


Red arrow charges the boot batt (+) and green is for the starter batt (+). Ground is, well... Ground (-)... Starter batt ground is convenient. Charging one does not charge the other...

You can charge both if you shunt (short) between red and green. But, needless to say, I wouldn't leave them shorted or run with the two connected... Who knows what evil lurks in the hearts of Mercedes...

Although I've never tried it, after a $130 tow charge (dead starter batt), it occurred to me you could probably give yourself a jump start by shorting red and green in a pinch!

Regards... Mark

pretty sure within its own electronics they can connect - does it not say in the merc PDF in post #11 if needed it will pull current from the rear to start the car

I can confirm^. Just be careful attaching anything to the red arrow post. It's very close to the fender. I can see accidentally touching it while attaching a charging cable and seeing the 4th of July.

That would be a handy feature BOTUS, but I don't think it works that way...

If the on-board electrical system battery is discharged, the battery coupling relay (F32k1) installed in the front prefuse box (F32) is closed when the vehicle is started. As a result, consumers required for starting the vehicle are also supplied with power from the starter battery circuit.

So, I think the starter batt can fire the essentials normally powered by the rear batt, but not vice versa (rear batt can't start the car)... Hence my thought to jump the red and green arrowed posts to jump start the car???

Regards... Mark

umm, that was the bit - but more subtly in the way is doesn't work than my cursory glance had spotted - seems a bit dim if the rear had any grunt, why it couldn't help sort the front too

BOTUS, thanks for the technical sheet on the 2 batt system. Solves the mystery of what it is doing what, and what it is Not! Also, a classic example of over-thinking by the Merc engineers. And, why they dumped the needlessly, over-complicated, 2 batt system in short order…

BUT, speaking of the rear batt… I finally replaced mine after 17 years of service.

The boot batt was still functioning and held a 12.4 V charge overnight, but was almost flat after a couple of weeks with no charge or start…

I shopped online for a shippable batt, and came up with Group 49 H8 as a replacement. But, when checking for fitment, only a few, in the $250 price range, claimed to fit a 2007 S600.

Anyway, I got a Weize AGM through Walmart (shipped UPS by Weize to the front door, about $150 including tax, and no core charge-- and, you can get a $10 gift card from O’Reilly’s, etc, for recycling the old batt). It was not supposed to fit the S600. But, better spec’s than the original MB batt:



I’m not familiar with Weize, but they make lithium and AGM batts with claimed months of low self-discharge (only 20% discharge after 12 months for the subject AGM!).

Perfect fitment:



Although, I’ll say, this was one of the hardest batt replacements I’ve ever done (“dead body in the trunk” style work area).

BTW, the rear cover, for access to the boot batt, has no fasteners! I took out the appox 1’ X 3’ floor panel, held with a couple of torx screws on the floor, and was able to get my hand under the vertical rear cover from the bottom. Once you get your hand behind the rear cover, you just “rip” it out. And, you just pop it back in when finished. I’m assuming my rear cover hadn’t been off since it left the factory… This was the first time I’ve ever seen the boot batt… And, probably, the last…

To continue the "needless rubbish from experts", attached is MB's technical on panel removal... Feel free to ignore it!!!

Regards… Mark

make sure u fit the vent right angle bit....


its uncomfortable but you can force you hand between the side trim and the battery and fit it easily - can't see it in place at the min from your photo - goes in a hole in the side (right) of the battery

Yes BOTUS, I refitted the vent tube. No small feat, along with the rest of the removal and replacement. Thankfully, it seems to be a once-in-a-generation necessity, given the boot batt's longevity!

Which begs the question: Is replacement by also removing the right trunk cover easier? Possibly. We've done it without the extra panel removal, so have the know-how. But, I wonder if, on first attempt, there would be less frustration by removing the right panel??? IIRC, the right panel is the source of all the obstacles...

Regards... Mark

top tips for battery removal -
hold the silly cloth strap up and pin out the way
pin up the earth lead - out the way
after taking off earth, undo the 13mm nut holding the short positive cable from the control model (then there's no fighting the stupid thing getting the battery out)
as you first drag battery forward, reach in the big easy gap and pop off the vent tube
don't do it on a hill (my drive is and the heavy b***er likes to stay put)
have the boot floor trim in place after loosening clamp
place a thick plastic bag (bigger then foot print of battery) - and slide the battery on to that - then it scoots across the floor to lift out easy peasy

as for right side trim - its a fight to refit (chicken and egg problem with rear boot trim) and you have to fight the nasty big plastic trim from the center boot catch area to take out the side trim. so best leave in play