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?
Mar 29, 2026 at 10:57 AM
I have a code saying that my auxillary battery is bad or not charging?
CHRIS1977IMPALA1
- MEMBER
- 3 POSTS
What to check next
Diagnostic guidance. Suggested checks and possible repairs are separate from the owner's reported results.
Obtain the exact fault code, the module reporting it and the conditions when it appears. Identify the auxiliary battery's role and the applicable charging/control circuit before selecting tests. Have the battery test report and relay-test method reviewed, then investigate the relevant connections and voltage supply under the specified conditions. The recorded voltage alone does not settle the cause, and the thread contains no evidence proving an ECU connection fault.
Guidance references and applicability
2CarPros: Original discussion 1712452 https://2carpros.com/questions/i-have-a-code-saying-that-my-auxillary-battery-is-bad-or Source for the owner's reported history and outcome. Contributor suggestions are not independently confirmed diagnoses. No attachment content was verified. Fluke: Diagnosing voltage drops https://www.fluke.com/en/learn/blog/automotive/electrical-automotive-troubleshooting General support for testing electrical supplies and connections under load. Use the actual vehicle's circuit diagram and test limits; this reference does not diagnose the reported fault.
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
William
Mar 29, 2026 at 12:25 PM
It's a brand new battery
Mar 29, 2026 at 12:41 PM
And I had them test it before I purchased it, I think it might be the connection between the relay and the ECU?
Mar 29, 2026 at 12:42 PM
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
William
Mar 29, 2026 at 12:43 PM
🔧 Tools & Auto Repair Parts
Find automotive tools, diagnostic equipment, and replacement
parts for your next repair.
Shop on Amazon →
Paid affiliate link. As an Amazon Associate, we earn from qualifying purchases.
Discussion summary
I have a code saying that my auxillary battery is bad or not charging?
The owner reported an auxiliary-battery fault message or code despite a reading of 12.8 volts and a relay described as good. The owner later clarified that the battery was new and had been tested before purchase, then questioned the connection between the relay and ECU.
Reported outcome: No confirmed fault or repair result was reported.
The exact code, battery-test results and relay measurements were not supplied. The contributor's reference to engine-cranking capacity does not establish the function or correct test for this auxiliary battery.