What is the life-span of a LiPo battery for a Phantom 4? I have had mine for 21 1/2 years with very little use. I charged them the other day after a long layoff, and it took almost three days for them to reach full charge. Are they on their way out? Or was it just the long layoff?
If you stored them charged they may have incurred some damage. Ideally you want to store lipos charged at 3.8V (basically half charge) so they don't incur wear from being stored in a high energy state. Typically you should expect lipo batteries to last somewhere between 200-300 cycles under normal use, with quads we do some really high discharge for short periods of time occasionally which can wear more on the cells.
It could just be one cell in the series drifted down in voltage much more than the rest and so it just had a hard time getting them all balanced but just a matter of using them and seeing for yourself how long they last and how many mAh get put back into them (can use a kill-o-watt meter monitor power from the wall if your charger doesn't tell you the current count/capacity)
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Honestly I end up smashing or ripping up the packaging on my cells (or over discharge) before they get a chance to die a natural death.
