********WARNING, OPINIONS EXPRESSED HERE ********
the short answer is,
It all depends on whether you intend to ever use them again.
discharging to too low a level or too fast, or attempting to charge too fast are
good ways to kill the batteries
the longer answer is,
some packs can't take more than 1C charge rate, and some can
the realistic answer is,
As long as it takes at a safe charge rate.
I rarely charge above a rate of 0.75C, and they are always done in 3 hours or
I usually charge at a rate as low as 0.25C and it rarely takes over 5 hours,
But then, I do NOT try to drain every last watt from my packs before I quit
at a 1C rate, on a pack discharged to the nominal discharge voltage, somewhere
around 1-1/3 hours should get it done
I do NOT charge batteries where I fly. I charge at home, take enough batteries
with me to "get my fix" and quit when the batteries are done.
It's the whole "Pay me now or pay me later." philosophy.
If you fry a pack at the field trying to get "one more flight", it is going to
cost just as much to buy a replacement as it would have cost to have an extra
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