SL/R230: 2003 starter battery not charging
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2016 E400
2003 starter battery not charging
I replaced both batteries after car sat for a year or so. the red battery light is on and when I checked the batteries I found the front battery isn't charging. Where do I go from here, thanks!
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2016 E400
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2005 R 230 SL350 (M112 3.7). Sold the 1966 W113 230SL recently
Presuming to gave your car fully charged new batteries, what made you conclude that the starter battery isn't being charged?
To be certain about those two fuses, unfortunately you have to take disconnect them. In the system, voltage may be on either side of them.
To be certain about those two fuses, unfortunately you have to take disconnect them. In the system, voltage may be on either side of them.
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From: Melbourne, Australia
2005 SL 500 R230
It can (probably) be only either relay K57 or the battery control module.
By the look of the circuit K57 should kick in in when the car is running and therefore charging both batteries. That relay is a known trouble spot.
Clear the red code and see if it comes back, but pull that relay and test it on the bench.
By the look of the circuit K57 should kick in in when the car is running and therefore charging both batteries. That relay is a known trouble spot.
Clear the red code and see if it comes back, but pull that relay and test it on the bench.
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SL500
Not true! One side of Fuse 1 is connected to +ve of starter battery, and other side of F1 is connected to BCU DC-DC converter (battery charger) and so would also have +12V, even if fuse blown.Therefore always 12 (approx) on both sides of F1, even when blown.
To test fuse you need to measure the fuse with Ohm meter, after disconnecting one terminal.
If F2 is blown, then neither battery would charge.
What voltage did you get on both batteries with engine running?
Rear battery is charged directly by alternator, like conventional single battery system, so you will see voltage rise when engine running to 14V or so.
Front battery is charged by the BCU, which contains a DC-DC converter, which acts as a battery charger, so you would not necessarily see a higher voltage when engine running. The DC-DC converter is controlled electronically by BCU
Neither relay will prevent the front battery from charging!
To test fuse you need to measure the fuse with Ohm meter, after disconnecting one terminal.
If F2 is blown, then neither battery would charge.
What voltage did you get on both batteries with engine running?
Rear battery is charged directly by alternator, like conventional single battery system, so you will see voltage rise when engine running to 14V or so.
Front battery is charged by the BCU, which contains a DC-DC converter, which acts as a battery charger, so you would not necessarily see a higher voltage when engine running. The DC-DC converter is controlled electronically by BCU
Neither relay will prevent the front battery from charging!
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From: North West England
SL500
K57 is an "emergency" relay. If one of the batteries is low voltage the BCU will energise it, connecting both batteries together, so hopefully there is then enough power on both batteries to start the car. You will also get consumers offline alarm.
This relay should normally never operate.
It gives issues when there is a parasitic drain on the rear battery, so it operates repeatedly when starting the car. Every time you see "consumers offline" message it has operated and bridged the two batteries together.
BCU Charges the front battery with a built in charger circuit, controlled electronically from BCU.
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From: Melbourne, Australia
2005 SL 500 R230
Ah, I see. I was just looking at the circuit and wondering what that relay does. From the circuit it appeared that it clicks in when the car's running, charging both batteries in parallel.
It might be stuck closed though, with fused contacts, bypassing the BCM and throwing the code. IIRC there were posts on that.
Then the OP's issue is most likely the BCM, since that's the only other path to the starter battery. But first, clear the code and if it doesn't come back, then check to see if the batteries are charging.
It might be stuck closed though, with fused contacts, bypassing the BCM and throwing the code. IIRC there were posts on that.
Then the OP's issue is most likely the BCM, since that's the only other path to the starter battery. But first, clear the code and if it doesn't come back, then check to see if the batteries are charging.
Last edited by Tom Manning; Nov 9, 2022 at 06:38 PM.
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SL500
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From: Netherlands
2005 R 230 SL350 (M112 3.7). Sold the 1966 W113 230SL recently
New batteries throwing a code is not uncommon. Don't fix it if it ain't broken is the first option for the OP to consider.
If the code can be cleared and the starter battery keeps a healthy charge after several starts there's nothing wrong with that car.
If the code can be cleared and the starter battery keeps a healthy charge after several starts there's nothing wrong with that car.
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From: Melbourne, Australia
2005 SL 500 R230
Ah, you mean OP wouldn't necessarily see front battery charging? I'm not sure that's right; I assume it's charging all the time, like the rear; the voltage on the front battery should definitely come up to about 13.8 Volts IIRC within 5 or 7 seconds; mine definitely does.
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2016 E400
Not true! One side of Fuse 1 is connected to +ve of starter battery, and other side of F1 is connected to BCU DC-DC converter (battery charger) and so would also have +12V, even if fuse blown.Therefore always 12 (approx) on both sides of F1, even when blown.
To test fuse you need to measure the fuse with Ohm meter, after disconnecting one terminal.
If F2 is blown, then neither battery would charge.
What voltage did you get on both batteries with engine running?
Rear battery is charged directly by alternator, like conventional single battery system, so you will see voltage rise when engine running to 14V or so.
Front battery is charged by the BCU, which contains a DC-DC converter, which acts as a battery charger, so you would not necessarily see a higher voltage when engine running. The DC-DC converter is controlled electronically by BCU
Neither relay will prevent the front battery from charging!
To test fuse you need to measure the fuse with Ohm meter, after disconnecting one terminal.
If F2 is blown, then neither battery would charge.
What voltage did you get on both batteries with engine running?
Rear battery is charged directly by alternator, like conventional single battery system, so you will see voltage rise when engine running to 14V or so.
Front battery is charged by the BCU, which contains a DC-DC converter, which acts as a battery charger, so you would not necessarily see a higher voltage when engine running. The DC-DC converter is controlled electronically by BCU
Neither relay will prevent the front battery from charging!
Thread Starter
Junior Member
Joined: Jan 2022
Posts: 35
Likes: 16
2016 E400
Not true! One side of Fuse 1 is connected to +ve of starter battery, and other side of F1 is connected to BCU DC-DC converter (battery charger) and so would also have +12V, even if fuse blown.Therefore always 12 (approx) on both sides of F1, even when blown.
To test fuse you need to measure the fuse with Ohm meter, after disconnecting one terminal.
If F2 is blown, then neither battery would charge.
What voltage did you get on both batteries with engine running?
Rear battery is charged directly by alternator, like conventional single battery system, so you will see voltage rise when engine running to 14V or so.
Front battery is charged by the BCU, which contains a DC-DC converter, which acts as a battery charger, so you would not necessarily see a higher voltage when engine running. The DC-DC converter is controlled electronically by BCU
Neither relay will prevent the front battery from charging!
To test fuse you need to measure the fuse with Ohm meter, after disconnecting one terminal.
If F2 is blown, then neither battery would charge.
What voltage did you get on both batteries with engine running?
Rear battery is charged directly by alternator, like conventional single battery system, so you will see voltage rise when engine running to 14V or so.
Front battery is charged by the BCU, which contains a DC-DC converter, which acts as a battery charger, so you would not necessarily see a higher voltage when engine running. The DC-DC converter is controlled electronically by BCU
Neither relay will prevent the front battery from charging!
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From: Netherlands
2005 R 230 SL350 (M112 3.7). Sold the 1966 W113 230SL recently
I think (not: I know) that the car must be driven for a while, so the system(s) can normalize. If indeed the starter battery gets no charge you will be able to start the car many times.
Did you clear the code? Does it come back?
Did you clear the code? Does it come back?
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2016 E400
