How long can a LiPo pack be kept fully discharged before storage charging?

Question

There is a reason LiPo packs are recommended to be stored at a storage charge. It is commonly known that storing a battery fully charged reduces its capacity and performance. What about storing batteries discharged after flights for extended periods of time?

Has any research been done on this topic?

Suppose I flew today and the next time I'll be flying in 7 days. Which is better:

1. leave the packs discharged, and charge after 7 days,
2. storage-charge immediately, fully charge after 7 days,
3. fully charge immediately, leave charged for 7 days?

What about 3 days or 2 weeks instead?

Answer 1

The most appropriate answer I could find comes from this study which states that a charged LiPo should be returned to storage charge if you do t plan to use it within the next 12 hours, and leaving it longer will cause a buildup of damage. The study is very interesting, though it may not explicitly answer your question I recommend giving it a read.

Answer 2

Like the other answer, I couldn't find a specific, good study to go off of for any sort of real statement. The ones I have seen tend to be pretty low sample sizes and somewhat limited scope. What I did find that supported some experimental data directed at the type of lipos that get used for quadcopters was this: https://www.propwashed.com/lipo-storage-voltage/.

The TL;DR from that is the study did not find any real difference between a lipo stored at any normal voltage level.

With that said, I have seen another study someone did with a slightly larger sample size (5-10 batteries over several months), but I can't seem to dig it up right now. The conclusion that the study had (from my recollection) was that storage at full (4.2v) or empty (3.5v) had an impact on the battery similar to single-digit charge cycles (fully charging, then fully discharging the battery) per week. Take this with a grain of salt, but from my memory, a fully charged battery stored for a week was roughly equivalent to 2-3 additional charge cycles, while a fully discharged battery was roughly equivalent to 5-6 charge cycles.

My personal approach is to not worry about it too much. The only battery I've seen significant damage to from storage was left at empty for almost a year and had discharged well below 3v/cell. Typically I charge my batteries to fully charged if I'm planning a flight session in the next couple days, and storage charge them otherwise. This is less than ideal if I needed absolute performance out of them, but I'm not a pro racer who needs that, and essentially all the batteries I've retired have been due to crash damage, not degradation over time.

To that end, I'd recommend whatever best fits your flight style. Most data I've seen shows a lipo will generally last 300-400 charge cycles, so cutting that by 25% due to keeping them at full charge all the time seems like a reasonable trade-off if it means you're more easily able to get out and fly when you want to. If you're in a situation where you can easily charge from storage->full prior to ever flight session, keep them at storage and enjoy the extra lifespan.

Answer 3

It depends what you mean when you say fully discharged. If you mean fully = 3.5v / cell then I wouldn't leave it all that long. But you shouldn't be flying your packs down to this level if you want them to last.

These days, I always try to land with 20-30% left. It sounds like a lot, but it will prolong the life of your packs enormously. Eg landing a 1300 pack at 1000mAh consumed.

So if you come home with all your packs at 3.65-3.7v / cell I wouldn't worry about storage charging them for even several weeks.

My understanding of storage charge has always been that it's a non-degrading ongoing charge designed to prevent your batteries self-discharging into damage land over LONG periods of time.