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?

  • A picture of the battery + terminals would help narrow down what might be going on here...
    – Zaid
    Jan 31, 2017 at 19:31
  • A coating of the anti-oxidation grease commonly used in electrical panels works wonders at keeping your battery terminals happy. I don't ever have to clean them over the life of the typical battery unless something happens to remove the grease.
    – Perkins
    Feb 1, 2017 at 1:56
  • jump it again and measure the battery voltage while its running. should be 13.8-14.+
    – agentp
    Feb 1, 2017 at 4:20

2 Answers 2


I see two possibilities here:

  1. The battery is weak
  2. 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.

  • The terminals are pretty corroded after a little over a year of use. I'll post a picture when I get home. Jan 31, 2017 at 19:40
  • 1
    @Zaid "Given that the battery and alternator tested fine" - or, the guy at AutoZone measured the voltage straight after switching off the engine, got 12.6V, and said "that's good enough for somebody who only wants a free battery check."
    – alephzero
    Jan 31, 2017 at 23:08
  • 1
    I too am suspicious of the quality of the supposed battery and alternator check. It could have just checked that the resting voltage of the battery was over 10 volts and the running voltage was over 12. That hardly rules out a battery problem. (Or an alternator problem for that matter.) Jan 31, 2017 at 23:52
  • Right. 12v should be just good enough, but the fact you got 10v immediately after is suspicious. Try measuring the voltage at the alternator and see what it's producing.
    – atraudes
    Feb 1, 2017 at 0:35
  • My line of thinking was that if it's not being charged properly due to voltage drop across the terminals, there's little to be concluded from any voltage measurements.
    – Zaid
    Feb 1, 2017 at 2:54

Best guesses:

  1. 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.

  2. 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.

  3. 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.

  • The cables are corroded but not loose. The battery is 14 months old, so not very old. I am reading the voltage directly off the battery. The different voltages were read while the battery was not under load. I'll inspect the cables next. Jan 31, 2017 at 22:52

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