# For how long should I charge my dead car battery without damaging it?

I drained by accident my car's battery. When I connect it now to a multimeter it shows 3V. I connected the battery already to a trickle charger and I would like to know for how long I should charge it, so that I don't damage it by overcharging. E.g. can I measure with my multimeter a voltage or current level reached and then know that I have to stop the charging? Also, can I use my multimeter to calculate in advance the charging time? Tnx

Quantities I know:

• The trickle charger's output is 13.0V and 10A
• the dead car battery showed 3.0V before charging
• the battery has a capacity of 60Ah

EDIT: I made a mistake with measuring the current of the charger. The right current (or at least the one stated on the charger itself) is not 0.3A, but 10A

• How are you measuring the charger current? I assume you put it directly across the charger leads? That is risky, It would be best to connect it in series between charger and battery to see what it actually puts out when charging. The 0.3A looks reasonable, but 3.0 on the 200 mA range does not seem right. You may have blown the internal 200 mA fuse. Commented Aug 22, 2022 at 18:56
• @PStechPaul You are right, I made a mistake when measuring the current. When I figured this out and tried to redo the measurement I blew a fuse on my multimeter :) The charger itself states that the current is 10A (sorry for the confusion, my garage has no lights and I didn't notice) Commented Aug 23, 2022 at 6:30
• If you know what you are doing and have access to a CC/CV supply, run it with 0.1 A current limit and as high voltage as you power supply allows, say 30 V until CC kicks in, then 14.4 V and 1 A until you are above 10 V where you can increase the current. Commented Aug 23, 2022 at 7:09
• Most of these charger states quite high current 4A, 5A, 10A, but these current rarely are present during the whole charge process. Most of them give those current during an hour or even less before going to the real long term current they can give. For example my many car battery charger stating 4A or 5A give around 0.8A to 1A during steady states charge. Which is way way way too low to charge car batteries which are often more than 40Ah Commented Aug 23, 2022 at 8:02
• With a very discharged battery, trickling isn't the best move. It's like getting a car moving, the first push needs to be harder than subsequent ones. So, a big boost, to get it back to around 12v will get it working again. After that, it can be trickled for as long as you can - several days has usually worked for my batteries, even ones that haven't been used for 12 months. But make sure the electrolyte is topped up, otherwise it's a fool's errand.
– Tim
Commented Aug 23, 2022 at 16:19

A typical Pb-Acid car battery is charged with around 13.8V. So 13V will be fine. A battery trickle charger is a voltage source, meaning that as the battery voltage rises, the charger puts less current into the battery. When the battery voltage equals the charger, there is no significant current going into the battery. You can keep most trickle chargers connected indefinitely to a car battery.

When I've had car batteries get discharged this low, I have put 50mA to 150mA on them for several days (could be a week), and the batteries actually recovered and continued working for several years.

The things to watch for are:

1. Do you smell sulfur while charging?
(battery could be permanently damaged)
2. Is the voltage slowly rising?
(check it once a day or so. If it isn't battery could be permanently damaged)
3. Make sure the water level is full in each cell (use distilled water if possible)
• thanks a lot! I can tell the following: 1) I definitely smelled something when I went into the garage after 90 min charging, but I am not sure it was sulfur. I thought it might be ozone, which I considered normal for charging devices. 2) I measured the voltage of the battery and it showed surprisingly 11V after 90 min charging! 3) Is it possible to open all car batteries? I think mine is not manufactured that way. I would appreciate if you have further comments, especially on 2). Commented Aug 22, 2022 at 16:23
• Yes, it takes a while until the battery becomes "hungry" again and the voltage seen at the trickle charger is falling. You can increase the charge current in this case.
– Jens
Commented Aug 22, 2022 at 16:42
• @NeStack If it's sealed, then the best you can do is try to recharge it and then "load test" it. Most chain automotive stores will test it for free. Commented Aug 23, 2022 at 2:36
• 3V? It's dead Jim. Commented Aug 23, 2022 at 7:22
• @NeStack Sulfur is a rotten egg smell. It is not subtle. Commented Aug 23, 2022 at 14:27

If you took it all the way down to 3V, it's probably already significantly compromised. I know they're expensive, but you really should consider replacing it. The sulfer in the acid precipitates onto the plates in a hard crusty form. It is possible to "renew" a lead acid battery by performing a "wakeup". You fully charge and discharge the battery about 3-5 times, and (if it hasn't gone too far and you're kinda lucky) you'll bring it mostly back. But don't discharge to 3V! You would go to maybe 10V.

Anyhow - You might put it in the car and it might work. But consider it's rather warm right now. I don't know where you live, but if it's someplace that has cold winters you definitely should consider replacing it.

• You don't know where OP lives, but do know it's "rather warm" there - how do you manage that? Commented Aug 24, 2022 at 6:17
• @TobySpeight Seriously? OK you're right. You got me. I subconsciously assumed he/she is part of the 80% of the worlds population that lives in the northern hemisphere where it is summer right now. My bad. Very sorry. :( Forgive me? Commented Aug 24, 2022 at 6:44

I would say toss that battery and get a new one unless you don't have strong preferences whether it drops you or not. Typically when they get discharged down that far they don't come back much, if at all.

You could try adding distilled water and charging it, and then putting it on a measurement system (I believe some automotive parts stores will do a quick test for free- but of course you might suspect they would tend to want to sell you a new battery) to see what CCA remains.

Also many modern chargers refuse to start charging a battery that has been discharged that low (because the algorithm says to throw that battery away at that point), so a bench power supply or an old-school transformer/rectifier charger can be used to bring it back to within charging voltage.

My perspective is that a cost of a hundred dollars or three is not worth the risk of being left in a dark windswept parking lot in a Canadian winter. In some places the risk of being dropped in a high crime area might be a consideration. It's also an easy DIY swap in most cases (well, unless you have a modern BMW etc. in which case you have to get into the computer using OBDII and some software and 'register' the battery telling it the capacity and type such as AGM or flood).

Of course, by 'toss' I mean return for proper recycling of the lead and disposal of the acid.

• Having left the interior lights on for a week or so on occasions, and come back to a very flat battery, I'd be well out of pocket by 'tossing' and buying new! See my comment under the question.
– Tim
Commented Aug 23, 2022 at 16:16

The battery is fully discharged, so to get back to 60 Ah storage, you need to put at least 60 Ah in. If the charger supplies 0.3 A, then that will take 200 hours. If it works. This level of deep discharge may well have permanently damaged the battery and the capacity may always be lower than you'd like.

I don't imagine a 0.3 A charge to 13V will damage the battery if left indefinitely.

• Thanks for the answer! Surprisingly, after 90 min of charging the battery shows 11V! Do you have an explanation why? Commented Aug 22, 2022 at 16:24
• The voltage isn’t proportional to the state of charge. Unless the battery is almost completely discharged you’ll see about 2V per cell or 12V total.
– Frog
Commented Aug 22, 2022 at 17:59
• This doesn’t necessarily mean the battery is ok though. If left discharged, the battery’s axis will deposit lead sulphate on the plates which decreases the battery’s capacity. There are devices (pulse chargers) that claim to be able to reverse this process but little evidence that they work. So you may find that your 60Ah battery is now a 10Ah battery, only time will tell. Either way, you can charge it up to about 14V without damage.
– Frog
Commented Aug 22, 2022 at 18:04
• (axis -> acid, presumably, a typo) Commented Aug 22, 2022 at 22:29