My girlfriend's 09 Camry is not starting unless it gets jumped. She took the car to Autozone where they tested the battery and alternator and said both were fine. I measured the voltage across battery posts to be 10.6V. After jump starting it and running it for a few minutes, we turned the car off and measured the voltage across posts to be 12.6V. We tried to start the car again but couldn't. I measured the voltage one more time immediately afterwards and it was back to 10.6V. What causes this? It seems like the battery is obviously not working but what exactly is causing these voltage changes?
I see two possibilities here:
- The battery is weak
- The battery terminals are corroded enough to cause voltage-drop issues, so both battery and alternator are fine but
- the charging system isn't able to provide full voltage to the battery
- likewise the starter motor isn't getting the expected voltage from the battery
Given that the battery and alternator tested fine, I'm inclined to think that you need to replace the battery terminals.
One or more cells of the battery is low on water or the plate is collapsing. How old is the battery? Near end of lifespan? A battery can read full voltage, but under load will drop voltage. Are you taking the voltage directly off the battery itself or the cable clamps? Drops to voltage off the battery itself would indicate the need to check the battery first.
Bad connection to starter or problem with starter. Are the cables corroded or loose? Does it take a lot of spinning to start the engine? If the starter is eating more power than normal it can drain the battery. Especially if it is shorting down.
Breaks in the battery cable. Lots of corrosion can eat away the copper. That and it can insulate the connections at the battery/starter/ground sides. That wouldn't allow enough current to flow normally.