2007 W221 s550 starter battery not charging
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Newbie
Joined: Jul 2020
Posts: 5
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From: Vilnius
W221 s550
2007 W221 s550 starter battery not charging
Hy all, have a problem here. Long story short. I had a flat battery in the trunk for electrical equipment. I opened car manually start normal, went to garage and checked rear battery it was hot and some acid around, guess battery shorted inside. Pull ot out and replaced with a new one. Went for a couple hours drive in highway, after about 1.5hour drive stopped for a smoke, came back to a car and it didn't start looks like starter battery flat, jumped positive wire to starter battery and trunk battery, started right away. Finished my trip, shutdown the engine and tried to start no luck. Charged the front battery manually and been driving for a one week until it got flat again. Connect MB Star diagnostic tool, went to actuations and activated battery coupling relay, connect multimeter at this point it was charging, but as soon as i turned off star it opened again. Was reading around and found that starter battery is charged by some other module next to the trunk battery. Some dc/dc charging converter. Can someone point out what to check next.
Member
Joined: May 2011
Posts: 87
Likes: 15
S65 E320
did you find out what this was, i charge mine and it soon drains, i put multimeter on it and it is not receiving charge from the alternator, whereas the starter battery in the ending bay is. This battery is draining when the car is off and not charging when on. Is there a faulty relay somewhere that is discharging
Newbie
Joined: Jun 2021
Posts: 1
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s550
did you find out what this was, i charge mine and it soon drains, i put multimeter on it and it is not receiving charge from the alternator, whereas the starter battery in the ending bay is. This battery is draining when the car is off and not charging when on. Is there a faulty relay somewhere that is discharging
Newbie
Joined: Sep 2024
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From: Aldershot
Mercedes 2007 S 500 w221
Did anyone get to the bottom of this issue. I have a 2007 S500 with an aux battery in the boot and a starter battery under the bonnet. When the engine is running the starter battery is reading circa 14 volts but the aux battery is only reading 12 volts and falling. This leads me to believe that the starter is receiving a good charge from the alternator but the aux battery isn’t.
the box under the bonnet next to the battery seams to have a relay in it which i thinking could be the split charger. Anyone able to confirm?
my thoughts are if this has dirty contacts inside then this could be the problem.
I’ve tried testing the relay and it seams to engage when a voltage is applied…
any help gratefully received.
the box under the bonnet next to the battery seams to have a relay in it which i thinking could be the split charger. Anyone able to confirm?
my thoughts are if this has dirty contacts inside then this could be the problem.
I’ve tried testing the relay and it seams to engage when a voltage is applied…
any help gratefully received.
Out Of Control!!
Joined: Apr 2019
Posts: 11,922
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
AUX not charging...
Check in your trunk, near AUX for a red 7.5A fuse... its the protection supplying inbound charge power.
It goes open when AUX is drained and charge by ALT is attempted on a shorted batt.
Use a float charger to maintain unused vehicles
It goes open when AUX is drained and charge by ALT is attempted on a shorted batt.
Use a float charger to maintain unused vehicles
Last edited by CaliBenzDriver; Sep 14, 2024 at 07:45 PM.
Newbie
Joined: Sep 2024
Posts: 10
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From: Aldershot
Mercedes 2007 S 500 w221
Thanks. I've checked the fuse and it was fine. Interestingly when i fully recharge the aux battery via mains charger the cigar lighters work. It appears that only when the aux battery is low does the car then begin shutting down bits of the car that it doesn't need.
Out Of Control!!
Joined: Apr 2019
Posts: 11,922
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
When available power is low, the car computers work together to cutoff secondary consumers. This is done to preserve essential driving functions.
Using "battery FLOAT charger" will help prevent high Amps damages - See AMZ for a winning selection.
Last edited by CaliBenzDriver; Oct 22, 2024 at 10:39 PM.
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Newbie
Joined: Sep 2024
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From: Aldershot
Mercedes 2007 S 500 w221
I still have the same problem with the alternator not charging the aux battery though....
I've done a continuity test on the yellow / red (single wire) wire coming from the relay in the F32 box next to the starter battery under the bonnet and going to yellow / red (single wire) in the rear power distribution unit and the test fails. Does anyone know if this is the same wire? With the engine running the wire give a reading of circa 5v.
If i take this same red / yellow wire from the relay and touch it to ground then the relay engages and the aux battery starts to charge.
Any ideas??
I've done a continuity test on the yellow / red (single wire) wire coming from the relay in the F32 box next to the starter battery under the bonnet and going to yellow / red (single wire) in the rear power distribution unit and the test fails. Does anyone know if this is the same wire? With the engine running the wire give a reading of circa 5v.
If i take this same red / yellow wire from the relay and touch it to ground then the relay engages and the aux battery starts to charge.
Any ideas??
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Out Of Control!!
Joined: Apr 2019
Posts: 11,922
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
I still have the same problem with the alternator not charging the aux battery though....
I've done a continuity test on the yellow / red (single wire) wire coming from the relay in the F32 box next to the starter battery under the bonnet and going to yellow / red (single wire) in the rear power distribution unit and the test fails. Does anyone know if this is the same wire? With the engine running the wire give a reading of circa 5v.
If i take this same red / yellow wire from the relay and touch it to ground then the relay engages and the aux battery starts to charge.
Any ideas??
I've done a continuity test on the yellow / red (single wire) wire coming from the relay in the F32 box next to the starter battery under the bonnet and going to yellow / red (single wire) in the rear power distribution unit and the test fails. Does anyone know if this is the same wire? With the engine running the wire give a reading of circa 5v.
If i take this same red / yellow wire from the relay and touch it to ground then the relay engages and the aux battery starts to charge.
Any ideas??
It seems the relay is not being commanded.
What controls the AUX relay is a SAM.
Last edited by CaliBenzDriver; Oct 23, 2024 at 03:39 AM.
