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Thanks for the link UT. I was hoping for a bit more detail if possible. Are they wired in parallel or does each battery have a function.
The R231 has the main in the trunk and secondary behind the passenger seat correct?
I notice on the consumption panel there is a graphic for battery and it lights up in a pretty non-standard manor. Sometimes its most green under coasting and other times its under breaking like an MGU-K.
Is there a best way to charge them?
I believe the light is intended to show transmission function, at least it is that way on my R231. A green “D” means you are literally coasting, not accelerating or braking. I believe a bad or discharged battery light would be red in color.
If I keep a MB for over 5 years I have the batteries automatically changed out. The search function on this forum will show you hundreds of posts on the subject including getting the safety cage off the battery behind the passenger seat. Also Google “Why MB has two batteries” will give many web pages in the subject.
Last edited by Utopia Texas; May 18, 2022 at 06:02 PM.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Originally Posted by octony9717
Is there a best way to charge them?
Just wondering.
Check your owner's manual. I believe it states to charge them using the terminals in the engine compartment. @rmorin49 had a thread going a short while ago because he found battery charger leads on the battery in his trunk that were added by the previous owner. If you are going to use a trickle charger, make sure you know what you are doing.
In my previous SL500 (R230) I added semi-permanent leads for the battery in the trunk since when that dies, all of your "electrical consumers" shut down.
There are a few long discussions about the batteries in this 231 thread - rather than repeating it all here, do a quick search. AFAIK no one had yet seen a wiring diagram and there is a lot of discussion on wiring, charging etc etc on the batteries.
The green bar that shows up is regenerative braking.
FWIW AGM does not like “trickle charging”. Better to charge and leave and then top up as needed.
MB recommend (in the owners manual) use of the jump start posts under the hood for charging. No need to muck about with connections in the trunk or on the Aux battery.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
From what I can find on the subject, AGM batteries do need to be trickle charged, but you must use a charger designed for AGM batteries. Generally CTEK chargers are considered some of the best, if not the best, you can get. They are often relabeled and provided with certain high performance automobiles by the manufacturer.
From what I can find on the subject, AGM batteries do need to be trickle charged, but you must use a charger designed for AGM batteries. Generally CTEK chargers are considered some of the best, if not the best, you can get. They are often relabeled and provided with certain high performance automobiles by the manufacturer.
Yes a charger that is AGM compatible must be used.
MB chargers are rebranded CTEK and CTEK are good. I’ve had one since about 2008 that has been used and abused and it just keeps going.
Previous owner of my SL had installed a CTEK pigtail on the auxiliary battery but I put one on the main battery and have been using it. So far it seems to be maintaining both batteries.
Check your owner's manual. I believe it states to charge them using the terminals in the engine compartment.
On my R231 the jumping terminals are in the engine compartment. I have yet been able to locate the main battery though. It must be buried. My owners manual doesn't mention the location at all.
If you go into the boot / trunk and keep removing liners, you will get to the cranking battery (2 layers and some polystyrene fillers).
Pigtails are a waste of time & a placebo effect. Use the jumper points as recommended by MB and you will be fine.
I’ve been keeping my batteries healthy from the jump start points for 5 years now (19,000 miles). I had the battery tested (because I’m getting "blind spot assist" & "lane keeping assist” warnings) at a Varta (MB OEM battery manufacturer) authorised tester yesterday and he says the batteries are perfect.
I’ve never had a problem with my batteries even, with the limited use my car gets.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Originally Posted by ThatsMyDawg
Yeah I just purchased the replacements for the two batteries yesterday from MB. I plan to install them Sunday, if it is not raining.
The small battery is easier to access if you put the top down. That way you're not squeezing in the tiny space behind the passenger seat.
FWIW, I did not have a lot of difficulty removing the plastic cage on the aux battery. You just need to jiggle stuff around. I did find, however, that the aux battery I got had the terminals in a slightly different orientation and I could not get the protective plastic cage back on. So, I didn't.
All good info thanks. I’m going to start with the AUX and see if that helps. As mentioned, I had a Varta (OEM in my car) agent test mine (cranking battery) and they said it is fine.
Thankfully I was able to replace both of my batteries this morning. So glad I did it first thing, because it’s storming like no other right now. Everything went smooth and easy and was finished both in 30 minutes total.
Are you joking?
Yes. Soon as I was finished my wife and I took a drive out of town to have lunch with our daughter. A few hours later we returned. Later I went for a Walmart run and such. Now I am back home and all is well.
I even think my magic sky is quicker between transitions since the new batteries, but I can’t confirm that.
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