atikovi

New to me 2007 S550. It starts just fine but if I turn it off, after a few minutes, everything seems to be dead. The remote won't lock or unlock the doors or trunk. None of the doors pull-in closing works.When I open the drivers door, the gauge cluster stays off and the power seats don't work.As soon as I put the key in the ignition, everything comes back to life. But..............the cluster randomly turns off again in 30 seconds or a minute or two even with the ignition on. It was cutting out half a dozen times while trying to scan for codes. This doesn't happen when the car is running. And I left it unlocked and just now the alarm went off. If I disconnect both batteries, will that keep the alarm off? Any thoughts on the issue?

BOTUS

what size is the front battery? if its a small 35amp one then there's a massive one in the boot hidden - few people change either

these eat 37 amps with just the key on - very few batteries cope even when brand new -

atikovi

The front one is a Mercedes battery so it could be original? The rear is aftermarket. I'll test them and also connect a programming charger to see if these issues stop. I did notice the voltage drop to 11.79 volts while using the scan tool.

vettebk

Check the serpentine belt for alternator slippage.

atikovi

The small front battery tests good. The big one in the back wouldn't even turn the tester on. Will check with a voltmeter next.It is an AGM.

vettebk

When the big consumer battery in the trunk starts to fail, the car slowly turns off all electrical components due to low voltage. The big battery in the rear usually lasts about 10 years or so depending on how well maintained the car is. If your charging system is sub-par, the battery will die much sooner. You need 12.0 volts at rest. You need 14.2 volts while running. Anything less and you will have problems. Check that serpentine belt for any looseness around the alternator which would cause low charge. Keep us posted.

atikovi

3.6 volts on the rear battery. Will put it on a charger for now and see if it recovers.

vettebk

Very low. Incidentally, most trickle chargers won't charge when the voltage of the battery is lower than 9 volts. You may not have any success charging that big mother in the trunk. Some posters have pulled the cell covers and added distilled water to cover the tissue and then try to charge it. All other things being equal, I would just replace it for reliability.

BOTUS

charge them for 48 hours - then turn the key on and 20 mins later the volts will be under 12v again - they pull far too much current - the ONLY action is drive it or add a power supply if you need to do diagnostics

atikovi

Charger stops charging after 15 minutes as a safety measure. Battery is toast. Got an H8 in the basement that's been on a maintainer for years waiting to be used. Tests at 895 CCA. It's not an AGM though. How important is that? Really wondering how old the battery in the car is.

BOTUS

yep - my point its a BRAND new fully charged premium 850CCA 95 amp battery will still be flat inside 20 minutes......

vettebk

Not surprised at dead consumer battery. Could be due to age (probably) or it could be due to poor charging system. Swap that big mother and start it up and let it run. Better yet, take it for a ride for twenty minutes. Then check both running and non-running voltage. You need 12 at rest and 14+ running. If not, report back here.

Good luck. Keep us informed.

atikovi

My back gets sore just thinking about it, but will do.

atikovi

Took the battery out. Dated 5/23. WTH? Won't take a charge even out of the car.

vettebk

All the more reason to check the serpentine belt, belt tensioner, and pulleys for slippage on the alternator reducing the charge. Smart money says a battery really dated 5/23 was killed by a bad charging system. If belt is tight, check output voltage of alternator. Could also be a bad alternator.

vettebk

And if the charging system checks out good, and you kill this newer battery you replaced in the trunk; then you have a parasitic drain. But before we go there, let us know about the newer battery and its voltages at rest and running. We try to do a full diagnosis here.

atikovi

Newer battery at rest 12.5 V, with engine running 13.8 V. Seems to have solved the problems mentioned.

KolevBG

Hello! Im having the same issue with my car and i was wondering did changing the big battery in the truck solve the issue ?

atikovi

Yes it did.

KolevBG

Thank you for the fast reply Im going to try test the battery tomorrow hopefully the car unlocks

W210Driver97

did it solve your issue to ? Cause am having the same problem with W221 E350 2008. Nothing will work until key is put in the ignition.

BOTUS

you can't have a 221 E class

you can have

a 221 S class
or
a 211 E class

by 2008 its unusual to have a rear battery in the back of the S class

W210Driver97

It's an S class W221 S350 2008. Sorry typing mistake 😂
and yes it has a battery in the trunk.

Leadfoot61

All electrical in my 2008 CL550 W216 shuts off after a few minutes but I replaced my rear battery & it's fully charged. My front battery is new as well. Any suggestions?

BOTUS

as Mercedes cars age they all seem to get more voltage hungry - the likely explanation for that is more and more electronics ageing means magically (as designed) volts waste away and in worse cases modules forget to fall asleep as they were supposed too

most owners never have an issue if they drive at least 10 miles a week

but if you're a hoodie wanting to loiter about upsetting people and listening to crap on the radio when parked or the even younger generation wanting to view mind rot trash on video sources - go buy a power station and plug it in if expecting it to work for more than 3 minutes