What are the conditions for LiPo battery damage from undervoltage?

Question

It is widely known that Lithium-polymer cells are permanently damaged if their voltage drops below a certain threshold, around 3.0 volts, but I couldn't find much on the exact conditions and factors that regulate how this damage takes place (Which, I suppose, is related to the fact that few other applications dealing with lipos involve remotely as much recklessness as we subject them to).

The first of them is the contribution of voltage drop from internal resistance when the pack is under load. i.e. does the damage begin whenever a current (e.g. from a punchout) dips the pack's voltage, as observed from its terminals, below 3.0V/cell, or does the pack's "actual", resting voltage with no current drawn have to get that low?

Second, how fast does the performance of the battery degrade, and at what point does it stop? If voltage gets dropped (via IR) by a high current and it leads to damage, does it stop accumulating immediately as the current is removed and the voltage comes back up? What happens when a battery is discharged so that its resting voltage is below 3.0V, but is quickly charged back up or its voltage naturally bounces back somewhat?

Third, how significant is the time spent by the battery below the threshold versus the very fact of dipping down, i.e. what difference does it make if one rushes to charge an overdrained pack back to an acceptable voltage right after ripping it out of the aircraft (probably from another pack), versus doing it after getting home?

And finally, how does previously sustained damage from over-discharge contribute to further damage?

Note: Some context regarding how this kind of damage relates to degradation from the other abuse that we subject our cells to would be very much appreciated, although the main focus is still the undervoltage-related damage itself.

Answer 1

Your questions are challenging to answer because (my opinion) I think there is a lot more that goes into a batteries longevity than voltage staying between a certain threshold. In addition to keeping the voltage in specified ranges, the following also apply.

• Do you fly in hot or cold or humid conditions?
• Do you fly hard and fast all the time where your batteries come off the drone warm or hot, or do you take it easy and just fly for leisure?
• How long do you let your batteries sit below or above storage charge (3.8V/cell)?
• Do you charge your batteries immediately after using them or do you let them sit to cool down a bit (15 minutes or more)?
• What brand of battery do you have? Some are known to be junk, others have a great track record, but some bad batches now and then.
• What charger do you use? Is it reputable?
• Do you balance charge?
• How old are the batteries?
• How often have the batteries been crashed?

My opinion is, and I've been tracking battery usage since 2011, is that despite the claim of 300+ cycles, most LiPos will only last about 100 cycles due to the factors that I mentioned above, which also includes the occasional oops moments when you fly your battery below 3V/cell or on the rare occasion when you forget to put them in storage charge. On top of that, the number of cycles is highly dependent on the quality of the LiPo.

To make a long story short, a few mistakes with your batteries every now and then might "damage" them, but it shouldn't affect them as much as other factors involved in flying, like crashing and the others that I've listed above. Generally speaking, if you take care of your batteries, they should last you a while.

I know it's a vague answer that doesn't really hit on your voltage questions, but it's based on many years of experience, so I hope it helps you out.

Answer 2

I agree with the above answer. There are so many factors that effect the longevity of a battery that one can hardly pinpoint them all. I also believe that the specs on them are conservatively rated and they can often take a little more abuse than is commonly recommended. A single over or under charge as long as it's not extreme in nature is probably insignificant as the chemistry on not an exact science inside the pack.

I have found that as users of lipos there's really only a few things we can do to preserve them and get the most out of are purchase.

Here's my experience:

My home is my flying field so I used to like to fly very early in the morning as the lighting and wind was often most favorable. So, I'd charge my batteries the night before and have the ready at dawn when I wanted to fly. This resulted in my keeping them charged nearly all the time during the flying season. So, those lipos (3S 6400 Venom 20C) literally lasted 1 year and they started to puff and voltage sag. I never over charged them, and never 'undervolted' them. No physical abuse. Just normal flying, no crashes, damage etc. Just the charging habit was the sole factor in their short life.

I replaced them with 4 packs of the same quality, but instead NEVER left them full charged for more than a few hours, often not even that much. I also immediately storage charged them after use. I now charge them right before flying only and never the day before. the quad is the same, the conditions are the same and those batteries are now 3+ years old and none is puffed. They are starting to sag a little but still working. Will get this summer out of them then will replace. I also switched to a high quality real skyrc d100 charger replacing my copycat/chinese $12 clone cheapo knockoff so-called b6ac. Huge difference. The knock off was all over the place on the voltages and not accurate at all. Some charge cycles would come out 4.10v and others 4.35v with the same charge settings. You really get what you pay for in a balance charger. Get a good one.

Even if they don't puff, you can tell they're getting tired because they voltage sag. (3S will drop down into the 10's much quicker during flight so flight time suffers.)

I have often thought that running a parallel arrangement might be good as you could split the current load over more cells abusing them less. I've not tested the theory though. I get 17-23 minutes out of my current setup so really don't need to change much.