SL/R230: Red battery warning
Thread Starter
Member
Joined: Jan 2003
Posts: 185
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From: Cedar Park, Texas
2013 Porsche 911 Carrera 4S, 2004 SL55, 2012 Lexus IS250, 2021 Toyota Highlander
@mrvedit ... The 'U' cut. Nice. That makes things much easier.
Last edited by renncoder; Jan 14, 2026 at 07:19 PM.
Thread Starter
Member
Joined: Jan 2003
Posts: 185
Likes: 18
From: Cedar Park, Texas
2013 Porsche 911 Carrera 4S, 2004 SL55, 2012 Lexus IS250, 2021 Toyota Highlander
Super Member
Joined: Sep 2021
Posts: 541
Likes: 117
From: Ann Arbor, MI
2004 SL600
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Joined: Feb 2025
Posts: 141
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I’m just going to throw this out there, it might help.
On my wife’s 2023 SL 55, she kept getting the Low Battery Warning on her dash and MB app on her IPhone. I installed a Battery Tender on the 12V battery in the trunk. Apparently it keeps the 48V battery under the hood charged too, somehow. She hasn’t had any more dash or IPhone notifications.
The Battery Tender stays hooked up whenever she isn’t driving the car.
On my wife’s 2023 SL 55, she kept getting the Low Battery Warning on her dash and MB app on her IPhone. I installed a Battery Tender on the 12V battery in the trunk. Apparently it keeps the 48V battery under the hood charged too, somehow. She hasn’t had any more dash or IPhone notifications.
The Battery Tender stays hooked up whenever she isn’t driving the car.
Member
Joined: Oct 2020
Posts: 187
Likes: 41
From: Ottawa, Ontario
2013 GLK350 Canada
Many thanks @Mikes62 . I just got the auxilliary battery warning 3 times last night, so I'm going to start looking there. It's only about 3.5 years old and a Mercedes part, strange that it might have failed early. Could have been damaged in the alternator/starter battery fiasco I had last winter though. Seems like (according to the Doc I linked to earlier) it runs everything except for starting the car. Makes sense that it might be driving the Power Supply Control module nuts.
Unfortunate that the Engineering menu can't show the voltage of both batteries separately.
Unfortunate that the Engineering menu can't show the voltage of both batteries separately.
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Thread Starter
Member
Joined: Jan 2003
Posts: 185
Likes: 18
From: Cedar Park, Texas
2013 Porsche 911 Carrera 4S, 2004 SL55, 2012 Lexus IS250, 2021 Toyota Highlander
Senior Member
Joined: May 2020
Posts: 454
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From: North Carolina, USA
2004 SL500AMG, 2003 SL500 USA
Many thanks @Mikes62 . I just got the auxilliary battery warning 3 times last night, so I'm going to start looking there. It's only about 3.5 years old and a Mercedes part, strange that it might have failed early. Could have been damaged in the alternator/starter battery fiasco I had last winter though. Seems like (according to the Doc I linked to earlier) it runs everything except for starting the car. Makes sense that it might be driving the Power Supply Control module nuts.
Unfortunate that the Engineering menu can't show the voltage of both batteries separately.
Unfortunate that the Engineering menu can't show the voltage of both batteries separately.
Member
Joined: Feb 2025
Posts: 141
Likes: 47
Have the dealer do a “load test” on your battery. They can tell within a few seconds if your battery is still good or not. Amazon sells load testers for about $40 for your 12V battery. They are a good thing to have in your garage and can be used for all your cars.
Like mentioned earlier, if your battery sat in a low charge condition, it could be damaged. Have it load tested and get a Battery Tender hooked up. I have 11 Battery Tenders going at all times on toys and vehicles, boat, Harley, etc.
Like mentioned earlier, if your battery sat in a low charge condition, it could be damaged. Have it load tested and get a Battery Tender hooked up. I have 11 Battery Tenders going at all times on toys and vehicles, boat, Harley, etc.
Super Member
Joined: Sep 2021
Posts: 541
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From: Ann Arbor, MI
2004 SL600
Technically a "load test" was the very old way of testing batteries - a large heating element with huge cables was attached to the battery and it basically "shorted" the battery through the heating element for hundreds of amps of current while displaying how much voltage the battery could maintain at that current.
Modern battery testers are electronic circuits which measure the internal resistance in the battery in milli-ohms which accurately indicates the condition and capacity of the battery. Good ones are now very cheap and available on Amazon.
The YouTube "Project Farm" tester gave high marks to the "ANCEL BA101 Battery Tester". It is on Amazon for $34. I have bought several for family .
Personally I prefer the "TOPDON BT100 Car Battery Tester" which is a bit more expensive at $52.
As mentioned, extended or repeated low charge conditions will permanently damage a lead-acid battery. Some say below 11.5V starts damage, some say 10.5V. AGM batteries handle low charge conditions better than traditional lead-acid which is why MB specifies it here. Still allowing an AGM to drop below 10.5V is still going to damage it. This is why many/most of us keep these cars on trickle chargers when we are not driving them at least once a week.
Modern battery testers are electronic circuits which measure the internal resistance in the battery in milli-ohms which accurately indicates the condition and capacity of the battery. Good ones are now very cheap and available on Amazon.
The YouTube "Project Farm" tester gave high marks to the "ANCEL BA101 Battery Tester". It is on Amazon for $34. I have bought several for family .
Personally I prefer the "TOPDON BT100 Car Battery Tester" which is a bit more expensive at $52.
As mentioned, extended or repeated low charge conditions will permanently damage a lead-acid battery. Some say below 11.5V starts damage, some say 10.5V. AGM batteries handle low charge conditions better than traditional lead-acid which is why MB specifies it here. Still allowing an AGM to drop below 10.5V is still going to damage it. This is why many/most of us keep these cars on trickle chargers when we are not driving them at least once a week.
Thread Starter
Member
Joined: Jan 2003
Posts: 185
Likes: 18
From: Cedar Park, Texas
2013 Porsche 911 Carrera 4S, 2004 SL55, 2012 Lexus IS250, 2021 Toyota Highlander
So since I'm in the process of cracking the code of what's wrong with my car, I should keep the batteries on a tender?
Member
Joined: Feb 2025
Posts: 141
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Just use the alligator clips on the Tender at first. If it works, then you can attach a pigtail to the battery terminals.
My wife’s SL 55 has the 12V battery in the trunk. Thats the battery I attach the Tender to.
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