The keys in my car were left in the on position for about 12 hours (and maybe the interior light left on) and the battery is drained.

Is there any way to tell if I should replace the existing one or it should be recharged?

5 Answers 5


The battery should simply be recharged. Draining a regular automotive battery is not good to do on a regular basis, but doing it once isn't going to have a noticeable ill effect.

Giving the battery a full charge would help it, and it's more than most people would do. Most will just jump start the car, and hope for the best from alternator charging. That should work too, but a full charge with a charger is the best option.


A reasonable answer depends on how old the battery is. The expected lifespan of a lead acid battery is about 4 years. If your battery is nearing or over the 4 year mark, it would make sense to replace the battery as part of your standard maintenance cycle anyway.

Putting it on a slow charge overnight / over a day wouldn't be a bad idea, at any rate, even if you're over the 4 year mark... might as well try and eke out as much life from the battery as you can.


It depends on the health of the battery before it was discharged and how long it was discharged.

Working at a shop, we would top off the cells with distilled water, charge the battery overnight, then test the battery to see if it was still good.

You can either charge it (2A overnight or 10A for about 1-2 hours) or jumpstart the car and run it or drive it for about 15 minutes. Afterwards, you want to test how strong the battery is. Parts stores will usually test the battery (best way to tell). Alternately, you can try starting the car ~10 times to see if the starter turns strong or starts getting weaker. Try this test again after the car has been sitting a few hours to make sure it can hold the charge. If it starts sounding weak, replace the battery.


It depends - If your car is a second hand one it might need replacement. I used to own a Nissan Micra, and the battery was wiped out due to a similar accidental leaving of keys in the car with the lights on. The Micra's battery was never the same after that - already not brilliant at holding charge, leaving it to go flat was the nail in the coffin that killed it.

Charge it fully as @JPHi1618 suggests, observe the battery for the next few weeks, and if you notice a severe deterioration in performance you'll probably need a new one.


There are battery load testers at most shops to check if the battery will maintain the correct voltage. You just need to connect each terminal and hit the "load" switch on the device. A good, charged battery should remain in the green (good) section. You can also check the charging system with this tool.

But as long as the battery is somewhat good quality just charging it should be good enough. A slow charge is the best option for battery health.

  • Recharge before load testing too :)
    – rogerdpack
    Commented May 25, 2023 at 20:56

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