This sounds very suspect - anyone have insight into the components and how this system works? I’m using a CTEK charger connected as the manual suggests for jumping ( red positive terminal under the lid and brass ground stud). It started at 25% and moved to 75% after 8 hours. In the last 26 hours, it stayed at 75% and wont go to 100%. My MB Service Advisor mentioned the connections were correct and that there were 2) 12V batteries and a 48V. Any ideas?

 

I have a cheap battery charger from auto zone and have also noticed this behavior. I believe the normal but I don't know why. I believe the battery feeds to the 48v battery and vice versa and when charging it with a battery charger it feeds the 48v battery which then feeds the 12v battery as necessary. When pulling up the workshop menu to check the 12v and 48v battery, the 48v battery always tops off around 75% when the battery charger automatically thinks the battery is full.

 

On my wife’s 2023 SL 55 that we bought used a year ago, we had the same problem. It has a 12v battery in the trunk and a 48v battery under the hood. She was constantly getting Low Battery messages on the dash and on her IPhone.

I made a pigtail for the 12v battery and leave the MB tender connected whenever the car is not being driven. She doesnt get any more warning notifications.

I would leave your tender attached and it will probably go to 100% in the next few days. Apparently the Ctek tenders condition the battery and that takes a while.

 

Trickle chargers only dump a few amperes per hour into your battery, which probably has 80 to 100 amperes capacity, and the charge rate decreases as the battery charges. A badly depleted battery can take a couple days to fully charge with one of these glove-box size chargers. As others have said, putting the charger on while parked in your garage is helpful, especially if you drive infrequently and/or make mostly short trips.

 

Wow…. But that sure makes sense. Anyone have a high level diagram of the components in this system?

 

One more point -- these newer cars suck power (usually only about 50 milliamps or thereabouts) even when parked (unless you put it into sleep mode). Doesn't sound like much, but 50mA comes to 1.2 amp-hours every day. If your 100 amp-hour battery is sitting at 80% (80 amp-hours), you can run it down to 20% (where it likely won't start) in 6 to 7 weeks. The battery management system should shut things down before the battery is completely dead, but you're still stuck with a car that won't start. I plug in my trickle charger anytime I'm parked for more than a week.

 

Don't know about your specific cars, but I have "low charge" on my C300 if I don't drive it for a few days. MB said I should drive it every day because they "require" that you do so. And not just around the block. That said, I went to Hawaii for 10 days and it started fine when I returned. As for that sleep mode, my car won't enter it unless I've just come back from the dealership after they've reset my system. After that, no matter how much charge I have, it won't enter it. I also bought a decent charger, but when the car is in "low charge" it takes all of 15 minutes to get the charger to full charge it. These systems are a bit wacky. Love the car, though.

 

Ya gotta love it. Car manufacturers heap all kinds of "fuel conserving" technology, like ***, mild hybrid, and camtronic into their cars, and then tell you to go burn off a couple gallons just to charge the battery. They're completely insane.

 

AND - lets not AFM / DFM from GM that deactivates 4 cylinders on a V8. Nothing could go wrong there…. Seriously - we need to blame it on the regulators who want to force car manufacturer’s into this loose/loose box for a couple of MPG or emissions. This is all driving build costs up dramatically and those vehicles become maintenance nightmares when a sensor on one system gets dirty and the whole thing cant be diagnosed! They end up in the dump wasting all of those resources from vehicles that could still be working. Oh well.

 

This could be the behavior of a bad 12 V battery. When I had battery issues (https://mbworld.org/forums/c-class-w...ery-issue.html), I got the exact same behavior with the NOCO Genius 5 I have. After the replacement of the 12 V battery and an update to the EZS, I did a test with the charger and it now charges normally. Also, "standby option" works properly now.