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So, my car battery died 4 months ago while it was unused in my garage during the summer (interior dome light left on for months). I went to jump start it this morning, ran it 20km ( 30 minutes ) to work and after turning it off, the battery is 100% DEAD.

I can't even engage the door locks it's so dead. It appears that the alternator just isn't charging the battery at all.

Is the battery likely well and truly dead? If so, What killed it? Was it because it was lying dead for such a long time? Or would that make any difference?

I'm going to try charge it overnight. There are a few stores that do battery tests and will even charge a battery overnight for you (they want to sell you a new battery). I can try that, but I'd be curious as to the cause of this failure to charge so I can avoid similar situations in the future.

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    Don't have time for a full answer. Charge it on the lowest setting your charger has, for at least 24 hours. See my answer here for more information. – Move More Comments Link To Top Sep 21 '16 at 20:02
  • I charged a dead, dead battery with a "smart charger", and it took several days to charge, but when it was done it worked well for months after that. – JPhi1618 Sep 22 '16 at 17:01
  • @JPhi1618 Yeah, I may have done that if I had a charger at home, but the hassle just led me to buying a new battery. Most of the car battery chargers end up costing close to a new battery anyhow and it doesn't make sense for me to have one in the garage. – Trogdoriffic Sep 26 '16 at 19:48
  • Lead acid batteries 101. Guarantee it was dead for good and he bought a new one. – Jhawins Sep 28 '16 at 14:33
  • @Jhawins Good guess. And yeah I commented on the answer below that I just ended up buying a new one. Lesson learned for next time! – Trogdoriffic Sep 28 '16 at 22:11
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I'm not going to tell you to try and utilize a battery charger to fix the issue as @MoveMoreCommentsLinkToTop has suggested in the comments, but it has been my experience when a battery has sat for an extended period of time with a constant power draw on it, the lead plates get sulfide crystallized over and will no longer take a charge. If it does take a charge, it will be in a very weakened state where you may get 12vdc+ out of it, but the reserve capacity is shot.

Again, try the cheapest method first, which is throwing a charger on there at the lowest setting (a lot of chargers have a 2A setting, if so, us it). Do not be surprised if the battery is toast, though, and will need replacement.

  • Well, to be honest, I'm thinking it's an expensive mistake to make. Thanks for the update on WHY this happens. I find learning the "why" of things helps to avoid the issue in future. I guess when the battery first discharged, it's important to get it charged to full strength as soon as possible. This kind of problem seems like one that the car industry should have solved by now. – Trogdoriffic Sep 21 '16 at 20:49
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    If the vehicle is going to sit for any length of time, my best suggestion to you is to attach a battery maintainer (not a trickle charger) to the vehicle. This will keep the battery fully charged without the risk of overcharging it. – Pᴀᴜʟsᴛᴇʀ2 Sep 22 '16 at 1:44
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    I was going to go the inexpensive method... but in the end, when testing the battery only 5.5V were going through it on the initial test and figured whatever, I'll just get a new battery. – Trogdoriffic Sep 26 '16 at 19:45

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