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Old 05-20-2020, 06:01 PM
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Can running down a car battery perminently damage it?


So a couple of weeks ago I accidentally left the radio running in my car, and since I'm not going out much these days, I didn't notice it for a couple of days. Needless to say the car wouldn't start. So some time later, with some difficulty I managed to jump started it, drove about 20 minutes to the grocery store, noticed it was a bit sluggish getting started and drove it home another 20 minutes, figuring that 4 minutes of driving should be enough to give it a decent charge. But then when I tried to start it a couple of days later it wouldn't start.

Did my slow drain of the battery permanently damage it, or is there something else going on?
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Old 05-20-2020, 06:29 PM
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You have shortened its life. Car batteries don't like to be completely discharged.

Also, 40 minutes of driving won't fully charge a battery. Most cars also have a drain on a battery when off even if just from the clock in the dash or the radio. If you can get a charger on the battery and fully charge it, it may last you for a while still. How long depends on the battery's age and how many times it has been discharged.
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Old 05-20-2020, 06:37 PM
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It can happen--I have an old pickup truck with some weird unfindable ground side fault somewhere that causes it to go completely flat in about a week if the truck isn't driven. This is why I have a battery isolater on it now because those complete draining episodes have caused multiple batteries to fail such that they can't be recharged. The local autoparts store flagged my account and the vehicle because after the third round they wouldn't warrant the battery any more. Oops! So yeah, and if the battery has some time on it, it's more likely to happen.

And no, when a battery is completely flat, a few minutes is not enough drive time to charge it up enough to handle multiple starts in a short period of time. My son in law drives the pickup truck with the issue and after a week or so of COVID quarantine he got stranded at the local grocery store because he didn't isolate the battery--I told him to take the long way home, get on the freeway, put some time on the truck to get that flat battery charged up. Now he remembers to use the isolater lol.

Last edited by SmartAleq; 05-20-2020 at 06:39 PM.
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Old 05-20-2020, 07:28 PM
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A car battery is a lead plate and a lead oxide plate with an electrolyte made out of water and sulfuric acid in between them. This gives you about 2 volts (closer to 2.1) so a car battery has six of these cells, which is why it is called a "battery" and not a "cell" (though to confuse things we often refer to single cells as batteries as well).

When you discharge the battery, the lead turns into lead sulfate, the lead oxide also turns into lead sulfate, and the sulfuric acid turns into water. When you charge the battery, this reaction is reversed.

If you discharge the battery too far, the lead sulfate crystals become too large and solid to break up when you try to recharge the battery. This is often referred to as "sulfation" of the battery plates, and under normal circumstances, the damage is permanent. There are ways of getting the lead sulfate crystals to break up, but this isn't something that you can do at home.

If you have significantly drained the battery, you are better off putting the battery on a slow charger so that it can slowly try to turn the lead sulfate plates back into lead and lead oxide. This will lead to fewer permanent crystals forming and less damage to the battery.

Also, this doesn't really apply much to your case, but for the benefit of anyone else coming into the thread, it's better to recharge the battery (using a slow charger) as soon as possible after you notice that it's gone dead. The longer it sits in a discharged state, the more permanent the damage becomes.

But yeah, at this point your battery is probably toast.

You can try putting it on a slow charger and letting it sit for a couple of days to see what happens. You definitely have some permanent damage to your battery, but you might be able to recover enough capacity from it for the battery to remain useful for a while longer.
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Old 05-20-2020, 08:41 PM
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Sometimes a battery fails -- for example, the sulfation damages the lead plates, and the sludge shorts out a cell.

You find out that this is so by putting the battery onto a charger, and finding that it doesn't charge properly.

It's also possible to have a battery has become unbalanced -- one of the cells is flat. You fix this by overcharging a battery until it bubbles, then adding water to bring the levels back up again. You can't add water if you have a sealed battery: they do accept a very small amount of overcharge but if only one cell is much flatter the battery is dead.

On an old car, you needed to drive for a long time to charge a battery, because the charge levels were set low. On a modern car, you'll get quite a lot of charging in 20 minutes, typically enough to start a car (they're much easier to start than they used to be), but you need a long charge period if the cells have become unbalanced.
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Old 05-21-2020, 09:35 AM
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It's worth mentioning that car batteries, like brake pads and wiper blades, simply don't last forever, no matter how well they're maintained. In a way, OP was fortunate to have enough battery life left to still get the thing started.
Quote:
Originally Posted by engineer_comp_geek View Post
But yeah, at this point your battery is probably toast.
Agree. OP might be ok for a while but should really make plans to get it replaced, especially before any expected cold weather. They're not usually too expensive.

I had one fail dead one morning when it was -10F with an even colder windchill. Getting the battery hold down bracket nuts off and later on again just couldn't be done with gloves and my hands basically couldn't function for more than a few minutes at a time. It totally sucked. I'm forever grateful to my neighbor who let me use his truck to buy a new battery.
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Old 05-21-2020, 09:35 AM
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Thanks all for the information. Sounds like I need to find a place to get a new battery, assuming I can get it started again. I guess I'd heard that even if its not enough to fully charge a battery, running it for 15 minutes or so would be enough to start it again the next time. In the past when I'd left my lights on over night this would work. I wonder also if the fact that the battery was discharged very slowly over several days made things worse.
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Old 05-21-2020, 10:01 AM
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Alternators are not designed to charge batteries, they are designed to maintain the charge of a fully charged battery and run everything on your vehicles electrical system while the engine is running. I would suggest a good charge from a battery charger before buying a new battery.
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Old 05-21-2020, 11:04 AM
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I ruined a battery by not driving a car for several months. I went to the place where I originally bought the battery to get a new one. They looked me up in their system and said a new battery would be free, since it was still covered by the warranty. Being an honest guy, I sheepishly explained that it was my fault for not driving it, but they said it didn't matter, it was still covered. So if your battery is not very old, you may want to check with the place you bought it from and see whether the cost is fully or partially covered by the warranty.
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Old 05-21-2020, 11:26 AM
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Decent battery chargers are inexpensive enough that I think everyone should just have one in case. All the more so if you find there are times that you don't have a chance to drive much. Whenever I go on vacation or on an extended work trip, I connect a charger to my motorcycle and leave it in maintenance mode.
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Old 05-22-2020, 12:11 AM
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Quote:
Originally Posted by racer72 View Post
Alternators are not designed to charge batteries, they are designed to maintain the charge of a fully charged battery and run everything on your vehicles electrical system while the engine is running. I would suggest a good charge from a battery charger before buying a new battery.
Of course they charge the battery. It maintains a voltage higher than the battery's, usually 14 or so.
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Old 05-22-2020, 03:26 AM
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Quote:
Originally Posted by sps49sd View Post
Of course they charge the battery. It maintains a voltage higher than the battery's, usually 14 or so.
He didn't say alternators didn't charge batteries, he said they were not designed to charge [flat] batteries - with the part in square brackets implied by the context.
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Old 05-22-2020, 06:14 AM
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Modern car batteries only last 3 - 5 years. The last few that I have changed went from working perfectly to throw it out and replace it in very little time, twice instantly. Luckily there are folks that come to you and do the replacement where the thing gave up on you.

Last edited by don't ask; 05-22-2020 at 06:14 AM.
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Old 05-22-2020, 07:16 AM
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Quote:
Originally Posted by racer72 View Post
Alternators are not designed to charge batteries, they are designed to maintain the charge of a fully charged battery and run everything on your vehicles electrical system while the engine is running. I would suggest a good charge from a battery charger before buying a new battery.
I think it's more of a case that you can shorten the life of your alternator and battery then it 'can't do that', also the damage is done by now to both - so while you can get a battery charger on it, it won't help at this point, but could at a future point if it happens again. The re-charge of the deeply discharged battery from the alternator is not optimal for the battery's long term health, but is optimal to get the battery ready for it's next restart for the given output of the alternator. But in real terms that's how you would want this situation, it is a emergency recharge, where you want to get that battery charged up as rapidly as possible incase you would need it to restart the car if you turn it off. In that the alternator is designed to do exactly that, to get the battery ready for the next restart. In that if the battery is just a little discharged it's fine for the next restart, so it charges much slower.

If you have the equipment and time a battery recharger or new battery would be the best solution.
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