Fortunately, the ST-16 battery is lithium-ion, not lipo... so it does not have to be drained or stored at a specific level.. it can be kept stored at whatever level you choose. However to be sure you do not run out of power for the controller (which would be not so good, during flight), I'd recharge any time you get below 50%. Do not leave the charger plugged in for an extended length of time, after reaching a full charge.