Auxiliary battery in boot not charging
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From: Aldershot
Mercedes 2007 S 500 w221
Auxiliary battery in boot not charging
Hi all
please help my 2007 S 500 is having issue with the aux battery in the boot and I don’t think it’s receiving a charge from the alternator. The starter battery under the bonnet is good and is reading circa 14v when the engine is running.
the aux battery in the boot functions fine once charged via the mains charger but soon runs flat. Voltage at the aux battery when the engine is running is circa 12v hence me thinking it’s not receiving a charge from the alternator.
is there a fuse or split charge relay between the two batteries that I can look at??
all help gratefully received.
please help my 2007 S 500 is having issue with the aux battery in the boot and I don’t think it’s receiving a charge from the alternator. The starter battery under the bonnet is good and is reading circa 14v when the engine is running.
the aux battery in the boot functions fine once charged via the mains charger but soon runs flat. Voltage at the aux battery when the engine is running is circa 12v hence me thinking it’s not receiving a charge from the alternator.
is there a fuse or split charge relay between the two batteries that I can look at??
all help gratefully received.
Super Member
Joined: Dec 2018
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From: Washington, DC
2007 S600, 2007 Chrysler 300 SRT8, 2000 C5 Corvette, 2017 Mustang GT, 2023 300C, and a 2019 Lincoln
Sounds like a either a bad battery or a bad alternator (or a slipping belt). If you can check the date code on the battery, that will help. If the date is older than 7 years, I'd simply replace it. Several members have posts here about how to revive a glass mat battery if you want to spend some time researching it here. Keep us informed on what you do.
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From: Aldershot
Mercedes 2007 S 500 w221
Thank for the reply.
I’ve replaced the battery and fully charged before connecting and installing to the car.
the problem seams to be the car not charging the battery rather than the battery.
is there a relay, split charger or a fuse between the alternator and the aux battery.
I’ve replaced the battery and fully charged before connecting and installing to the car.
the problem seams to be the car not charging the battery rather than the battery.
is there a relay, split charger or a fuse between the alternator and the aux battery.
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Joined: Mar 2018
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2006 W221 S500L M273
A1 - Instrument cluster
A80 - Intelligent servo module for DIRECT SELECT
F32k1 - Battery coupling relay
G2 - Alternator
N3/9 - CDI control unit (with diesel engine)
N3/10 - ME-SFI [ME] control unit (with gasoline engine)
N73 - EIS [EZS] control unit
N82/1 - Vehicle power supply control unit
N82/1k1 - Quiescent current cutout relay
N93 - Central gateway control unit
LIN C1 - Drive train LIN
LW - Charge converter
B - Interior CAN
C - Drive train CAN
E - Chassis CAN
F - Central CAN
Thread Starter
Newbie
Joined: Sep 2024
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From: Aldershot
Mercedes 2007 S 500 w221
Thanks for the replies.
has anyone else experienced their auxiliary battery not charging when the engine is running that maybe able to point me in the right direction?
has anyone else experienced their auxiliary battery not charging when the engine is running that maybe able to point me in the right direction?
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S500
above the rear battery is a module - N82/1 - Vehicle power supply control unit - people here have claimed they die...
F32 is what I thought was a pre-fuse box - sits beside the front battery - had another guy claim he needed one..
F32 is what I thought was a pre-fuse box - sits beside the front battery - had another guy claim he needed one..
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Thread Starter
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From: Aldershot
Mercedes 2007 S 500 w221
Thanks for your reaponse.
ive spent the day looking at the power control module in the boot above the battery and when the engine is running there is only circa 12 volts in this area.
however, in the power distribution box under the bonnet next to the battery there is 14 volts on the battery under the bonnet (when engine is running) and it’s associated bits in the power distribution box. The aux terminals in the power distribution box only read 12 volts. Do you think the relay in the power distribution box could be on the blink. I’ve tried testing it and it does click but I’m thinking the internal contacts could be dirty and not allowing the aux battery to receive a charge.???
thoughts???
ive spent the day looking at the power control module in the boot above the battery and when the engine is running there is only circa 12 volts in this area.
however, in the power distribution box under the bonnet next to the battery there is 14 volts on the battery under the bonnet (when engine is running) and it’s associated bits in the power distribution box. The aux terminals in the power distribution box only read 12 volts. Do you think the relay in the power distribution box could be on the blink. I’ve tried testing it and it does click but I’m thinking the internal contacts could be dirty and not allowing the aux battery to receive a charge.???
thoughts???
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From: Aldershot
Mercedes 2007 S 500 w221
Yes. Multimeter across the aux battery shows less than 12 volts when the engine is running but the engine battery ( under the bonnet) shows 14 volts showing that the alternator is working.
my assumption is that the split charger or whatever it is that splits the charger from the alternator to the 2no. Batteries is faulty. Could it be the relay in the box under the bonnet by the battery?
my assumption is that the split charger or whatever it is that splits the charger from the alternator to the 2no. Batteries is faulty. Could it be the relay in the box under the bonnet by the battery?
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1983 Nissan Shltbox
Could just be a dead battery that the car will cut off from the charging system if it goes too flat. I would put a new battery in it if I didn't have access to more advanced (Xentry via Benzninja plug) diagnostic tools and go from there.
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From: Aldershot
Mercedes 2007 S 500 w221
Thanks.
I’ve tried a new battery but because it’s not charging then it just goes flat. I have also tried bench charging the battery but it still goes flat. It’s the car not charging the aux battery that’s the problem.
I’ve tried a new battery but because it’s not charging then it just goes flat. I have also tried bench charging the battery but it still goes flat. It’s the car not charging the aux battery that’s the problem.
