I've just been leaving my flight packs in whatever state. I pop one on the charger as I load the car. Then I pop another one on the charger before the first flight. The battery to use is always the one connected to the charger

I suppose if I had more than 3 or 4 packs, and I wanted to run each pack the same number of times in a day flying I'd run in to trouble with my system though. But at least I wouldn't try taking off with a flat battery .