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it's been a while! Hope everyone is well.

This might be kind of a dumb question but here we go:

My car's battery light turned on a few days ago, then suddenly went off one day when I rode the car to a close neighborhood. I figured it was fine and managed to drive there, turn off the car, turn it on and drive the way back with no issues. Next morning I tried to turn on the car and it would light the dashboard but not enough charge to turn the starter. I took the battery for a charge on a friend's mechanic shop and mounted it back to make a small voltage test. I checked the battery voltage (~12.8V) then turned the car on and checked again, noticing it actually dropped (12.2V). I figured it was the alternator, so I quickly turned the car off and disconnected the battery so I could keep most of its charge for a quick ride to the mechanics when I had time.

A few days later, I reconnected the battery only to be surprised that it couldn't start the car again! After this last turn of events, with the battery possibly discharging disconnected or having not been even charged properly on my friend's place, I'm tending a lot to think the battery is at fault and considering buying one already on the internet (cheaper than local shops).

Am I in the right track to assume that and skipping taking the car to a shop and just getting a new battery?

Some extra info: The battery came with the car already and I own it for about 2 years and some months. It had also discharged maybe a month ago because I forgot a headlight on for an hour, so it could have damaged an already tired battery? (I'm also suspicious that a healthy battery would discharge with only the headlights for 1h)

Thanks a lot in advance!

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    Did you put the battery in the car immediately after taking it off the charger? The 12.8 volts could then be the result of surface charge, giving a higher reading than it actually was at. If so then the battery wasn't truly fully charged which can make it more difficult to determine the root cause.
    – Mr. Spock
    Commented Mar 12, 2021 at 22:46
  • Not immediately, if I recall right it took a day after I picked the battery up at the shop to connect it to the car and make the voltage test
    – IanC
    Commented Mar 13, 2021 at 2:39

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Your battery got fully charged up to 12.8V and according to your comments it kept that charge for a day before you put it back into the vehicle to try and start it. This means that your battery didn't have a bad cell. This would lead me to believe that it could be the alternator.

However, after one start your battery's voltage dropped significantly and then couldn't start the car again a couple days later. This leads me to believe that the problem is indeed the battery. It is probably getting old and losing its ability to handle large draws (like starting the car) and to hold its charge. A good battery that your car is made to have would be able to start the car more than once even if the alternator is bad if you immediately disconnect it after the engine is started. I would recommend getting a new battery, especially since you say chances are it is getting old.

That said, if it is possible, I would also try and get your alternator tested. Most parts stores can do so. This will tell you for sure that the alternator is good/bad. But regardless it sounds like your battery could do with a replacement.

Best of luck.

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  • Thanks! I might have a friend coming over in 2 or 3 days and was thinking of using his car battery for a final test. But I'm very convinced it's probably the battery. I wish I had learned the tests I could have done before, could have done a load test on the battery when I started the car or something. It discharging by itself says a lot though.
    – IanC
    Commented Mar 15, 2021 at 3:06
  • Forgot to post the outcome: It turns out the alternator needed some repair, but probably since it wasn't charging the battery properly (also draining it) I think it damaged the battery as well which had to be replaced. It wasn't holding charge anymore.
    – IanC
    Commented Apr 13, 2021 at 21:16

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