I have a code saying that my auxillary battery is bad or not charging?

Tiny
CHRIS1977IMPALA1
  • MEMBER
  • 2003 MERCEDES BENZ E500
  • 5.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 280,000 MILES
I tested the auxillary battery with a volt meter and it tested 12.8 volts and I tested the relay and it's good, so what could be the problem?
Sunday, March 29th, 2026 AT 5:57 PM

4 Replies

Tiny
CANNON1349
  • MECHANIC
  • 906 POSTS
Hello, a multimeter is not a confident way of testing a battery. 12.8 volts tells us that the battery is charged, but not that it can handle the load of starting a vehicle. You can remove and take the battery to any parts store to have it tested, or of course drive the vehicle to those stores. As long as the battery is easily accessible for them they can test it in the vehicle.

William
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Sunday, March 29th, 2026 AT 7:25 PM
Tiny
CHRIS1977IMPALA1
  • MEMBER
  • 3 POSTS
It's a brand new battery
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Sunday, March 29th, 2026 AT 7:41 PM
Tiny
CHRIS1977IMPALA1
  • MEMBER
  • 3 POSTS
And I had them test it before I purchased it, I think it might be the connection between the relay and the ECU?
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Sunday, March 29th, 2026 AT 7:42 PM
Tiny
CANNON1349
  • MECHANIC
  • 906 POSTS
It's not uncommon for a new battery to fail a tes. If you can clear the code see how long it takes for it to return after clearing.

William
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Sunday, March 29th, 2026 AT 7:43 PM

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