One of the main reasons for topping off a battery before flight to100% is the rebalancing of the cells of the battery that occurs at the end of the charge. After a battery has started its automatic storage discharge, the cells quickly become unbalanced, as there is no cell balancing during the storage discharge. A variation in cell voltage between cells of a full 0.5V is very dangerous, as when any single cell voltage ever drops below 3.00V, the battery shuts off, to protect the battery, even if in flight. Always top off your batteries before flight, especially if there is any chance they have entered into the storage mode discharge cycle. Lesson learned the hard way myself, on a P3P that came crashing down 15 feet from me from 200 feet above me, 20 seconds into a test flight, on a battery that showed 85% charge, but had entered actually already into storage discharge, unbalancing the cell voltages!