I have a 2012 Toyota Matrix. It is very similar to Corolla.

The car struggles to start, and my first guess is to simply replace the battery.

However, when the car starts (means car alternator supplies electricity) most of the electrical components struggle to work. Here are a few examples:

  • Electronic Power System is not working. The steering wheel is stiff and hard to turn
  • Lights are dimmed
  • The Battery light indicator in the dashboard is on
  • The car stereo does not turn on.

I checked the belts when the engine is running, and the belt seems to be fine. Not sure if there is more than one belt on this car?

My understanding is that the car alternator must be able to properly supply electricity to all components even if there is a problem with the battery. Am I correct? That means a weak battery does not impact the functionality of car electrical components once the car starts and running. My concern is that something else (like alternator) is not working that causes the battery to drain. My guess is that the car could be running by the battery power right now.

Does above assessment makes sense and I should look for electrical problems somewhere other than the battery?

  • 1
    If the battery is bad it could be pulling the voltage down. Use a voltmeter to see what the battery reads when off and then see what it reads when running. If you have any local places that can test the battery you can try that before replacing it.
    – narkeleptk
    Nov 1, 2019 at 3:54
  • 1
    @narkeleptk, I did use a voltmeter to test the battery. It reads below 9 V. I though this could have related to defective alternator cannot generate enough power to charge the battery.
    – Allan Xu
    Nov 1, 2019 at 4:45
  • 1
    In addition to what @narkeleptk has already mentioned, it might be that a bad alternator damaged the battery by not supplying enough voltage/amperage. When the battery is constantly undercharged it becomes sulfated and can't recover under normal conditions, i.e. even if it gets enough voltage after an alternator repair. So even if the battery is bad and gets replaced it's a good idea to check the alternator too.
    – max
    Nov 1, 2019 at 4:45

1 Answer 1


If the battery is that low, then some alternators, even if they are working correctly, will not start to charge properly or at all.

Replace the battery with a known good one, then test the charging system thoroughly. That is the only way to check if the alternator is good. You could take the alternator off the vehicle and have it tested on a test bed but few places have those.

  • Can a bad battery cause the symptoms I mentioned above when the car is running?
    – Allan Xu
    Nov 1, 2019 at 15:23

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