Swarm them. When one is approaching 20%, send one that's charged to replace it. Once the replacement has arrived, return the depleted one to the charging stati

Swarm them. When one is approaching 20%, send one that's charged to replace it. Once the replacement has arrived, return the depleted one to the charging station and do not charge past 80% or so. If you make the charger smart enough to not completely charge the battery, and you have a ..eh.. Battery of copters available, you've solved the problem