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Battery issue

Tiny
DIVIDES
  • MEMBER
  • 2003 MERCEDES BENZ SL500
  • V8
  • 2WD
  • AUTOMATIC
  • 119,000 MILES
When I bought it went to pick it up and was battery was drained. Jumped it and battery light was on, thought maybe the alternator drained it, but once I drove it home 40 miles away with no issues. Parked it, and same story, not starting. Do these have two batteries? Is the main one under the hood or is there one that is responsible for starting the car only?
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Monday, January 3rd, 2022 AT 4:55 PM

5 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

As far as the battery is concerned, there isn't one designated to starting only. Is the alternator light staying on? For that to be on, chances are the alternator isn't doing its job. If possible, follow the directions in this link and let me know what you find:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

The link provides simple directions to determine if an alternator is performing or not. Keep in mind, the vehicle can keep running as long as battery voltage remains above a certain voltage. So, you may drive it and while driving, the electronics are drawing from the battery while the alternator isn't recharging it. As a result, it may not start the next time you try.

Try the test above and let me know the results. If you need help or have questions, let me know.

Take care,

Joe
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Monday, January 3rd, 2022 AT 5:41 PM
Tiny
DIVIDES
  • MEMBER
  • 898 POSTS
It only threw a battery light when I picked it up after I jumped it off, bit mot a red light. Went away after that, but there was a message that electronics are off to save power, which is typical on Benz. I just needed to know if there are other batteries such as the auxiliary battery that some Benz have. But I will check the voltage at alternator and battery. Thanks for your help.
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Monday, January 3rd, 2022 AT 5:51 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

First, I have to apologize. I found there was an option for this model to have a second battery. One is located under the hood and one in the trunk. Read pic 1

If you look at pics 2 and 3, they show the locations of the batteries. The battery for the starter is the one in pic 3, under the hood. The other is for onboard electronics. I suspect that is the one failing.

Again, sorry I made that mistake. Let me know if you have other questions.

Take care,

Joe
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Tuesday, January 4th, 2022 AT 4:58 PM
Tiny
DIVIDES
  • MEMBER
  • 898 POSTS
Lol, no problem. Yes, I found out earlier today and replaced both, both were weak, starting battery was at 3v. Thank you
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Tuesday, January 4th, 2022 AT 5:32 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

I'm glad everything was found. I went back into the schematics after seeing your post to me thinking, I guess there is a chance. After looking through schematics, there is a small label indicating an optional dual battery design. I got a knot in my stomach. LOL

Regardless, I'm glad to hear it's fixed. If you need help in the future, let us know. Just remind me that it has two batteries. LOL

Take care and I hope 2022 is a great year for you.

Joe
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Tuesday, January 4th, 2022 AT 5:46 PM

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