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How can I safely discharge a large lead-acid battery, like a car battery or UPS battery?

I assume I use a thick copper cord (I have that in the form of washing machine electrical supply lines, about a 1/4" thick) and then put a resistor in line. The problems I see with this approach:

  • How do I size the resistor so I am assured I am not discharging too fast and risking an explosion?

  • My experience is that large resistors are sick-o expensive. How can I get an appropriate resistor without spending a lot of money? Can I use graphite rod? (I can potentially get those cheaply) If so, how do I size the rod correctly?

  • How do I know when the battery is fully 100% discharged and completely safe?

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Assuming Ohm's Law work here; anything above 100 ohms should be not harmful. That's 100mA. 1 or 2W package will be necessary; if the battery is fully discharged you might have some hard time reviving it. – ammar.cma Jan 20 at 21:58
Frequently on this site, it is most effective to fully explain the situation upfront. You've stated your desired goal without first explaining your reasoning. Please explain your ultimate purpose - if you just want to hopelessly destroy the battery, a small hole in the bottom will do just fine. – Sean Boddy Jan 21 at 1:20
Deeply discharging a lead acid battery damages it so doing that for the sake of doing that doesn't sound like a good idea. And if you have some reasonable usecase for that then you'd better explain so that answers can address your actual problem. – sharptooth Jan 21 at 12:43
@SeanBoddy This is true for about any SE site, and is commonly called XY Problem – Dmitry Grigoryev Jan 21 at 13:50
A discharged lead-acid battery can hardly be considered safe. Sulfuric acid salts are pretty corrosive, and lead is a well known heavy metal. – Dmitry Grigoryev Jan 21 at 13:57
up vote 11 down vote accepted

Hook it up to a 60W headlamp bulb, that will take 5A. Car batteries usually have a capacity of around 45 to 60 AHr, if you assume the battery is fully charged and in good condition then it should take around 10 hours to discharge it.

I think that answers your question. But you should not fully discharge a lead-acid battery and leave it standing, you will permanently damage it.

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What do you plan on doing with the battery? If you completely discharge it, it will modify the electrodes and you will have to recondition it to get it back up and running. If you really want to discharge it fully, get a large resistor. Did you know a light bulb makes a great resistor? You can use any material just as long as it will conduct reasonably and it won't heat up too much. Or use a light bulb. 40W lightbulb/ 12V = 3A

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I don't want to re-use the battery. I want to FULLY DISCHARGE IT, is that clear? – Tyler Durden Jan 20 at 22:09
Why not take it to a recycling centre where it can be safely disposed? – Steve G Jan 20 at 22:40
@TylerDurden I think you should see a doctor. – Marko Buršič Jan 21 at 9:54
@TylerDurden its not clear, it could be inferred but no where in your question did you make a statement that you intended to discharge it fully. You said "How can I safely discharge a large lead-acid battery?" and "How do I know when the battery is fully 100% discharged and completely safe". You did not say, I need this battery fully discharged. A halfway discharged battery is pretty much safe as far as I'm concerned. – laptop2d Feb 5 at 22:45

To drain your battery, light-bulbs are (usual) a cheap power resistor (since linearity is not of importance).

The more bulbs in parallel, the faster the drain.

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beware. This solution is partially hazardous. Don't do this inside a building, near sources of ignition or delicate and expensive equipment. Do not breathe any developing gasses. If unsure, resort to the other suggested solutions.

Take a plastic bucket, fill with water, put two pieces of scrap metal (aluminium will do good) in it, wire them to the poles of the battery while taking care they do not touch each other. Iron is ok, but the residue is a dirty mess. Don't use copper as the outcome is toxic.

Then start pouring in some salt slowly until you have a sufficient reaction (gas production, but no excessive heat up). Don't breathe this!

This is a cheap and high performance load with integrated cooling device.

This will stop working once a certain voltage depending on the scrap metal pieces used is reached. After checking the voltage you can carefully connect the metal pieces under water. Wear goggles when doing this.

Last hint: do this outdoors only or you will blow up your house with the generated hydrogen.

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LOL. I know I shouldn't up-vote this, we're all professionals right? But you can have my +1 – Steve G Jan 20 at 23:26
The question is, if you did it indoors which would kill you first, the explosion? or the Chlorine gas? ;) – Tom Carpenter Jan 21 at 3:24
Never do this unless you have a good understanding about tge chemistry involved. – Max Ried Jan 21 at 5:53
This is about the worst suggestion you could give out, excluding a dead short. – Vladimir Cravero Jan 21 at 9:35
I would like you to think a little before writing, adding some real warnings at least. You say the Al leads should not touch, but did not stress that if they touch accidentaly they can melt together and possibly start a fire. Being funny is not required on this board, and being dangerous or uninformative just for lolz is much less than required. – Vladimir Cravero Jan 21 at 10:49

It appears that you have some erroneous, preconceived ideas about discharging a battery.
Specifically, if you want to fully discharge a typical car battery (12V, 60 A hr), all you need is a 20 ohm, 10 W resistor (or equivalent), and connect it across the battery terminals. Leave it connected for about 4 days, and with a voltmeter verify that the voltage is zero.
Since the current is only (12/20 =) 0.6A, no "heavy" wire is needed to connect the resistor.
Since the power being discharged is only (12 x .6 =) 7.2W, a 10W resistor should handle it with no problem.
Since it is drawing .6A, it should take (60/.6 =) 100 hours to discharge the battery.

The mentioned resistor should not cost more than $5 dollars. Good luck!

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