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I have a new five-month old (yes months) battery that is not keeping charge. I disconnected and charged the battery overnight using a trickle charger. I got the bad battery indicator on the charger. So, I disconnected and put it back on the trickle charger again. Overnight, the battery charged to 13.3 V. I let it sit another day and 22 hours later, the charge is 12.91 V. 48 hours later, the charge is 12.83 V. I guess I will check it every day for a week, but how much is a disconnected car battery expected to lose over a day?

Update: Ten days later, yesterday, it was at 10.39 V.

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  • Welcome to Motor Vehicle Maintenance & Repair! Feb 22, 2023 at 20:13
  • That's way too much discharge. You have a parasitic drain somewhere.
    – Huesmann
    Feb 26, 2023 at 19:05
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    It is a disconnected battery. Where could it get the parasitic drain from? Mar 4, 2023 at 18:35

2 Answers 2

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Typically, a resting battery loses about 5% charge per month. This is called "self-discharge".

I think what you are seeing is the battery, right after charging has a "little extra" charge in it, which quite rapidly goes down to what you are seeing after it rests. I would consider this an "overcharge" or something similar, where it comes back down to a "regular" voltage after charging. I don't know the technical term, but have seen this quite often when I charge batteries.

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  • Thanks, @Paulster2, I will report back on the reading tomorrow too. Feb 22, 2023 at 20:19
  • @user3236841 - You may see it drop a bit more, but realistically the voltage drop you're seeing isn't that much ... on .04 vdc. You might see it drop another 1/10 or two over another day, then settle out. Feb 22, 2023 at 20:29
  • I recharged the battery. Although, the trickle charger said "bad battery" it did report voltage of 12.79 V. The next day, it was down to 12.55 V. The third day it is at 12.05 V. Is this usual? Mar 7, 2023 at 3:38
  • @user3236841 - You need to take it to a battery place and get it load tested. It doesn't sound as though it is in good health. Mar 7, 2023 at 10:35
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I would get a better battery charger - they aren't expensive, especially a smaller one; the small ones work fine, but take a lot longer. I've used Battery Tender for years, and recently got a new one from Costco for $40 that charges at 4.5 amp max, which won't instantly start a dead battery, but after an hour or more, it will.

The problem with this one you used, aside from the false reading at first, is that it seems to be overcharging the battery. It should not charge more than about 12.6 volts.

Other than that, it sounds like your battery might be on its last legs. If it's normally parked at your home, with access to power, you could use a good trickle charger whenever you're not driving, and it would probably limp along for a while, but I wouldn't trust it 100% as it may die at any time.

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