BigTurboC4

Before I start, I know this has been covered a million times but I can't find an answer. The short story is that I was getting the typical low battery warnings on the dash and then no-start. AAA replaced the battery and things were okay for a month or two. Then the warnings came back so the voltage regulator (aux battery) was changed. The weird thing that's happening now is that if I'm sitting idle for 10-15min in park, I will get the "Stop car, leave engine running" warning light. It seems extra odd that this would come on while the car is running. In addition to that, if the car sits for two days without being driven, the battery will die. Is this another dead main battery (it will fully charge on a charger) or is something else going on here that needs investigated? Alternator, etc?

user33

Sounds like you're not getting any charging action. Measure the voltage across the battery terminals when the car is running. It should be over 13 VDC, typically 13.8 to over 14.

BigTurboC4

Okay so...

1.) Battery is NOT new as I'd expected. Date stamp is 3/21 which isn't super old but not brand new either.
2.) Voltage with the car RUNNING - 14.7v
3.) Voltage with car OFF - 12.29v

If I'm to believe the diagram below, it looks like the battery is nearing it's end however I'm going to "fully charge" it and see what it reads. If it charges to 12.5v+ and then immediately falls, that seems like there would be an issue.

W205C43PFL

I am not sure if I missed it from your post but did you ever try trickle charging it for a bit and see if it indeed is an alternator issue like you suggested?

nishalp

Dont think it would be an alternator issue if you're charging at 14+ volts while running. I know when you change the battery, you need to reset the battery info in the computer. Your computer uses this info to control the charging rates and functions. I found this link that might help also. May aslo be a drain somewhere. Steps in this article how to check for battery drain while not running.

Mercedes-Benz C-Class and C-Class AMG: Why Does My Battery Keep Dying? | Mbworld

BigTurboC4

I didn't see anywhere in there on how to reset the battery indicator so proper charging occurs. I also looked elsewhere and couldn't find much for W205 cars...any suggestions if this is actually necessary when replacing battery?

Happened again this week where the car was running, sitting at a red light and the "stop car, leave engine running" light popped up and that has me so confused.

W205C43PFL

The procedure can be done via XENTRY or with an advanced scanner capable of MB functions.

As for the procedure done in XENTRY, please kindly see attached, Step 13 confirm battery change in SAM control unit.

Edit to answer your question: Reset isn't a must but if you don't reset it, the lifespan of the new battery will be shorter as the alternator is still on full load mode when the new battery doesn't need such.

BigTurboC4

Thanks for that!

I've also just read that if a battery charger is connected to the body charging spots, the Xentry part is not necessary? Can you confirm whether or not this is true?

W205C43PFL

I would still go through the reset, honestly it is not a big deal in today's world to not have XENTRY, there are capable scanners like a Launch CReader Elite Benz V2.0 like forum members been recommending that can do the same thing.

https://mbworld.org/forums/e-class-w...-reader-2.html
https://mbworld.org/forums/e-class-w...view-link.html

BigTurboC4

Okay so here we are again. Second voltage converter is in but just for fun I wanted to see what the charging system was doing.

Car on. Battery voltage 14.7.
Alternator amps have dropped to 6.5 at idle and falling.

is this typical alternator behavior or is something going on here?