Accidentally drained OEM car battery

Car year is 2014 and so is the battery. I left the car key at on in the ignition overnight with parasitic draws (electronic devices on). In the morning, battery at rest voltage was 12.01 which is very low. However, the car started with no struggle and seems to idle normal. I kept the car at idle for a few min and checked the alternator output was 13.85, and turned it off. The battery at rest voltage went up to 12.60, and now it’s going down during the past 40 min or so, and it’s now down to 12.37. It may further go down. Should I get the battery charged as soon as possible, or should I first take the car out for a drive for 30+ min and see how it goes? Before this incident, the battery has been keeping very good at rest voltage at around 12.55 to 12.64. The battery is OEM and is almost 4 years old. How likely is it that draining one night have done it harm?

If the battery was able to start your car the next morning with no problems, I wouldn’t worry about it.

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As in one of your other threads you seem to have an obsession with battery voltage. The battery is 4 years old and my feeling is that summer heat will finish off a battery just at the wrong time . Why not just get a new one ?

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Running the motor will NOT recharge a battery that is as discharged as yours, contrary to popular belief. Charge it up ASAP since a battery left in a discharged state can quickly ruin it. Then get it load-tested.

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Can you explain why the car’s charging system won’t recharge a low battery? My cars have successfully recharged low batteries by just driving them, including a few times when the battery was so low that I had to start it by letting it roll down a hill and then engaging the clutch with the transmission in gear.


The alternator will indeed recharge a battery that’s down to 12v. But you are putting excess strain on the alternator and could hasten its demise, so it’s never a bad idea to charge a drained battery with a charger before you ask the alternator to overwork itself.


It’s simple math. A battery will only accept a certain charging rate regardless of alternator output. As it charges, that charging rate drops. If a 100 ampere/hour battery is half charged (12.1 volts for a 12 volt battery), it will not accept the amount of charge to bring it to full capacity in that short a period of time. The purpose of a high capacity alternator is to be able to run more/higher powered accessories in addition to the engine, not to provide super-fast battery charging.

This is why a pattern of frequent short trips will leave the battery chronically undercharged, leading to its early demise.

Well now that you’ve introduced the time variable, I agree. But before you said that the alternator will not charge the battery - sure it will, if you drive around long enough. Such problems are a good excuse to do the motoring equivalent of the pilot’s “$100 hamburger.” Just get in your car, drive somewhere far-ish away, have lunch, then drive home. It’ll charge as long as the alternator doesn’t get over-strained.


If you’re worried about overloading the alternator by having it charge up an unusually low battery, drive with all the accessories turned off. Headlights, heater/ac, etc., until the battery is reasonably charged up again or “one start discharged”.
Frequent short trips are not my problem, my commute to work is nearly an hour. That’s why my cars never rusted out their exhaust systems.

I had to Google the term “one hundred dollar hamburger” LOL, I’ve had plenty of hundred buck burgers because every time I ride a motorcycle just for fun, I always seem to end up at a restaurant far, far away.

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Glad to hear that your draining of your battery wasn’t on purpose. :wink:

I’m with Volvo on this one. Your battery is 4 years old. Depending on where you live (and the weather conditions in winter & summer,) I’d seriously consider replacing this battery before it leaves you stranded somewhere.
Here in the Arizona desert, I wouldn’t hesitate to change it out soon. 4 years here is living on borrowed time on a battery.


Some auto parts stores will perform a conductance test on the battery for free.

They can tell you if the battery requires replacing right-than-and-there.


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Thanks for all your opinions and advices. I checked again after work (that is about 10 hours have passed, and I drove another car and left the car in question in my garage all day). The at rest voltage is 12.27. It was at 12.37 the last time I checked in the morning. So my logic is it drained overnight to 12.01, and I turned the engine on for about 2 min and it read 12.60 after I turned off the engine, then 12.37 after 40 min later, and then 12.27 after 10 hours later so it seems to be holding the charge. It is still low, but it’s probably not too bad. I think it will fully charge once I drive it for a while. Further opinions?

This is the battery’s state of charge at a static voltage reading.


Yes, thanks for the chart so if it’s 12.27 at rest that about 60 some percent charged which not too terribly low. Given that is had drained to 12.01 overnight, and then losing only about 0.10 from 12.37 to 12.27 during 10 hours I was at work doesn’t sound like battery is quiet shot.


It’s saying the battery isn’t capable of holding a charge.


Sorry dude but you need to go back to math class. As long as there is enough voltage to excite the field coil, an alternator will put out a charge. If the output of the alternator is greater than the load on the battery, the battery will charge.

The voltage and current supplied to the field coil is controlled by a Voltage Regulator in older vehicles and by the Power Control Module (PCM) in most new vehicles and some, if not all will not allow current flow to the field coil if it does not detect a minimum battery voltage. That minimum voltage is usually around 6-8 Volts, not 12.1.

To the OP, my daughter also has a 2014 Toyota and yesterday morning, it would not start due to a dead battery. She didn’t leave anything on, it was just its time.

Well, I drove the car for exactly 12 min at no more than 25 MPH this evening following my last post. Immediately after I turned off the engine, the at rest voltage of battery was 12.83. It went down to 12.76 within a minute after that (this I don’t know if it’s normal). I guess, I’ll see how much it will hold in the morning, and go from there.

You are measuring the surface charge in the battery from being recently charged, not the actual state of charge. What you measured 12 hours later is the actual state of charge. It is normal for the battery voltage to drop some after being charged, don’t expect it to stay at 12.73 volts.

My car’s battery tends to measure around 12.5-12.6 volts in the morning before starting it. If it measures in the 12.0 volt range, it will still crank the engine but it has a slow crank, not the typical fast cranking when the battery is fully charged. If you notice the cranking is on the slow side, that’s a worry b/c that can damage the starter motor over time. So if you have slow cranking going on, good idea to follow the advice above and have the battery shop tested. Conductance test, load test, either will do. It’s not possible to make an accurate assessment of the overall condition of a battery by only measuring it’s unloaded voltage.

When a battery gets accidentally discharged like that, good idea to recharge it using a battery charger, rather than relying on the alternator. When that happens to me I charge it overnight at the 2 amp rate. I always disconnect the ground lead from the battery when using a battery charger, to minimize accidentally damaging something sensitive in the car’s electrical system.

Since you seem to measure the battery voltage quite often, one test you could do that would be a better battery assessment is to measure the battery voltage in the morning with nothing turned on, and then measure it again with the engine off and the headlights on bright. The differences in voltage between the two measurements would provide a pretty good measure of the overall battery condition. If you monitored that over time, you could have a pretty good idea of how your battery is holding up. As the battery degrades over its lifespan, that voltage difference will gradually increase.

Batteries seem to last a long time in the Buffalo NY region. My daughters 2010 Corolls and my 2012 Camry both have orighnal batteries and mine still tsts good. My 2002 Scotts lawn tractor I bought 4 years ago still has the battery that was in it and it sits in the shed 6 months at a time and I don’t even bother to disconnect a cable. Because of lake Erie it doesn’t get bitter cold here, I think -16 F is the coldest I have seen at my house.