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I have a deeply discharged battery(showing 0.49 volts or so) and the parts store have told me it is not charging after 3 hours so it’s not rechargeable.

In this answer the comments somebody mentions they left it for 24 hours with a typical charger and it did start charging: Car Battery won't charge after sitting for a Year Is leaving it for 24 hours likely to make a decision difference?

I have just seen this video https://m.youtube.com/watch?v=gMPuzmhYCQc where a guy hooks up a good battery(without a charger on it) up to a deeply discharged battery and apparently after 1 hour it’s at 12 volts. Is this a viable method to recharge a battery? It seems to have no charger involved which is even better than the one in the linked question which makes use of the charger.

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    Just try it... both methods, you will soon know and learn from the experience. – Solar Mike Sep 24 at 21:35
  • Battery charging is basically chemistry. If you need to break down some compounds that shouldn't be in the battery at all and took a year to slowly accumulate, don't expect to get rid of them in 5 minutes. But when they have been broken down, the battery will start working more or less normally again. – alephzero Sep 24 at 23:05
  • Modern battery chargers can be "too clever for their own good" for this sort of thing. They have too much protection to stop any potential fires and explosions. Hooking up another battery might cause a fire or explosion, but it won't "shut down and do nothing just in case". – alephzero Sep 24 at 23:09
  • @alephzero - I hook up two lead acid batteries in parallel all the time on a charge with zero issues. The point of doing this is just like you said, modern chargers are too clever. If they don't detect a good voltage source of near charging range (ie: somewhere above 10v), they won't charge the battery. By putting a second battery on there in parallel, you allow it to detect the source voltage so it will in turn charge both. This works great on dead AGM batteries. I'd have no issues telling others to do the same. – Pᴀᴜʟsᴛᴇʀ2 Sep 25 at 0:51
  • "too clever for their own good" > "I'm to sexy for my shirt". – Alaska Man Sep 25 at 17:44
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you can revive old batteries, I've done many times. Google "revive lead acid battery" takes a week, very low charging.

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Yes, hooking a deeply discharged battery to another good battery in parallel is a good way to revive the discharged battery. One reason that the discharged battery may not be charging is many battery charges have an internal safety mechanism that prevents charging a deeply discharged battery. This is because it wants to avoid potentially charging a battery with a short that when left unattended could cause big problems.

The way around this is to "trick" the charger by having it read a higher voltage on the battery. This is one by hooking the discharged battery to a good battery in parallel (positive to positive and negative to negative).

I found this description of this process on Optima's website here:

https://www.optimabatteries.com/en-us/support/charging/resuscitate-deeply-discharged-battery

They do a good job explaining the process as well as safety precautions. Note that this description specifically mentions AGM batteries, but this method works for flooded acid batteries as well. I know it works because I did it many times myself while working at a battery store.

RECOVERY OPTION #2: THE DIY SOLUTION FOR CHARGING A DEEPLY DISCHARGED BATTERY.

This is a recovery method for the do-it-yourselfer using the equipment you have in the garage. With this option, you're going to "trick" your traditional charger into charging the deeply-discharged AGM battery.

Here's what you need:

  • Battery charger (under 15 amps)

  • Jumper cables

  • A good battery, preferably holding voltage above 12.2 volts. (It can be an AGM or flooded lead-acid battery)

  • The seemingly dead, deeply-discharged AGM battery

  • A voltage meter

  • A watch or timer

Here's what you do:

Hook up the good battery and deeply-discharged AGM battery in parallel – positive to positive and negative to negative. Do not have the charger connected to the battery or turned on at this stage.

Next, connect the discharged battery to the charger, then turn on the charger. The charger will "see" the voltage of the good battery (connected in parallel), and start delivering current.

After the batteries have been connected for about an hour, check to see if the AGM battery is slightly warm or hot to the touch. Batteries naturally become warm during charging, but excessive heat may be an indication that there really is something wrong with the battery. Discontinue charging immediately if the battery is hot to the touch. Also discontinue the process if you hear the battery "gassing" — a hissing sound coming from the safety valves. If it's hot or gassing, STOP CHARGING IMMEDIATELY!

With your volt meter, check back often to see if the AGM battery has charged to 10.5 volts or above. This generally takes less than two hours with a 10-amp charger. If it has, disconnect the charger from the wall outlet and remove the good battery. Now, connect only the deeply-discharged AGM battery to the charger. Turn on the charger and continue until the AGM battery reaches a full state of charge (at least 12.6V), or until the automatic charger completes the charge process. In most cases, the AGM battery will be recovered.

Note: In the above process it says if you hear gassing stop immediately, this is true if your battery is AGM. If it is a conventional flooded acid battery a little gas is normal when charging, however if the battery is hot like it says and there is a large amount of gas escaping the same applies, stop immediately.

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