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Is there any research that shows the actual decrease in capacity/performance/longevity of high discharge current LiPo batteries, like those used in quadcopters, depending on the total time they spent being fully charged? Maybe there are some chemical processes that start to occur after some time of being charged which should be avoided and the time until they start being harmful can be measured or calculated...

What rule of thumb (that can be scientifically backed, or would at least be an educated guess) can we assume to keep the batteries in good shape without being too extreme?

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You should store the battery fully charged as little as possible; a few days is probably fine, but when it gets closer to a week or more you might start to see it's health degrading. That said, there is no hard limit where it will stop working, so my advice is to charge it when you are going to use it, and then charge / discharge it to 3.8 V / cell when you are done.

A battery sitting at room temperature fully charged will lose about 20% of its capacity in a year. A battery sitting at 60 degrees Celsius will lose 40% of its capacity in just 3 months.

https://brushlesswhoop.com/storing-fully-charged-lipo-batteries/

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  • $\begingroup$ That seems like what I usually try to do, but sometimes fail. I have seen the rule-of-thumb limit of 2 days myself. Some sources seem to suggest much longer limits though. Just to be on the safe side, I would probably stick to the lower ones myself, although 20%/yr compared to about 1%/mo @ storage (see Robin Bennett's answer) doesn't sound as bad as I thought... $\endgroup$ – Michał Trybus Apr 14 at 21:14
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4-max (a UK battery supplier) recommend storage charge for anything over 1 month:

If you plan to store your LiPo cells for an extended period (over 1 month), discharge them as you would normally. Then charge them to only 3.80V/3.85V per cell.

https://www.4-max.co.uk/pdf/prolong-life-lipo.pdf

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