SL/R231: Auxiliary Battery
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Member
Joined: Sep 2020
Posts: 165
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From: Stockport UK
SL 350 R231
Auxiliary Battery
There's that much information on here (which is a good thing) but I can't work out what the auxiliary battery actually powers. Some posts say it only provides power for the stop start, others I've read say it powers the roof. Can somebody explain please. 
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From: Brookshire, Texas & Cat Spring, Texas
2019 SL550/2023 EQS 580 SUV
I was told that in the design MB did not want any possibility of accessories draining the main battery which is strictly to start the car and nothing else. The auxiliary battery does all accessories. Naturally once the engine starts the alternator supplies power while recharging the batteries. My source was a salesman so take it for what it is! 😊
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From: Hagerstown MD
2015 SL400; 2019 X5M40i
I asked at my dealer. Service advisor told me one story, one of the sales guys told me another. I really would like to know for sure more out of curiosity than anything. I am using a CTEK on both batteries if the car is going to be parked for more than a few days.
Thread Starter
Member
Joined: Sep 2020
Posts: 165
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From: Stockport UK
SL 350 R231
Ok So if the Aux battery was to be disconnected would the car still start and run. I'm not trying to find fault here or anything , it's just that this is the first car I have owned that's got two batteries. As I may have said before it's a learning curve for me.
BL
BL
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Joined: May 2017
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From: Phoenix
2015 SL400, BMW Z3, Datsun 280Z, a couple of jeep Grand Cherokee's - And a Commander Airplane
The web is a wonderful thing!
This should explain it.
https://xenons4u.co.uk/blog/mercedes...ion-explained/
This should explain it.
https://xenons4u.co.uk/blog/mercedes...ion-explained/
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Thread Starter
Member
Joined: Sep 2020
Posts: 165
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From: Stockport UK
SL 350 R231
I'd rather disconnect the stop start by pulling the fuse but I think it's shared with other components. I don't have a problem with the Aux battery (apart from trying to get the dam cage off which I will master eventually) As I think it might have a lot of influence on other systems.
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From: Nothern Georgia, U.S.A.
'17 SL450, '25 Lyriq, '25 Sonata Hybrid, '96 Corvette GS coupe
Since the above link is rather generic, it's also rather vague for those of us concerned with the battery's exact function in the current R231. A fellow on another forum (jmattioni) researched the R231 auxiliary battery's exact function with M-B, and came up with the following.
It is referred to by M-B as the "ECO Start/Stop Auxiliary Battery". When the R231's start/stop function is active, the car's system switches all the "consumers" (i.e., loads) over to the auxiliary battery while the starter motor is cranking the engine. Once the engine starts, all consumers are switched back to the car's main system until the next start/stop cycle. This keeps a more constant voltage available for those consumers.
It is referred to by M-B as the "ECO Start/Stop Auxiliary Battery". When the R231's start/stop function is active, the car's system switches all the "consumers" (i.e., loads) over to the auxiliary battery while the starter motor is cranking the engine. Once the engine starts, all consumers are switched back to the car's main system until the next start/stop cycle. This keeps a more constant voltage available for those consumers.
Last edited by Dr. Manhattan; Nov 17, 2020 at 07:18 PM.
Super Member
Joined: Jul 2018
Posts: 563
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From: Nothern Georgia, U.S.A.
'17 SL450, '25 Lyriq, '25 Sonata Hybrid, '96 Corvette GS coupe
You may/may not have any idea how long we have been arguing/speculating about this subject, but it has been quite awhile...so many thanks to jmattioni for finally chasing this down for everybody!
Last edited by Dr. Manhattan; Nov 17, 2020 at 07:20 PM.
