Battery size and the level of charge will dictate the charging time, but it is important to note that even for a completely drained battery, charging does not happen in a linear way. At first they charge very quickly and then the rate slows down to a trickle. We recommend charging to approximately 90% total charge to minimise total charging time and to preserve battery life. On the fast charger that ships with the Flame series and Inferno (available as an optional accessory for other models) when 90% charge is reached, all the lights will flash. If you leave the batteries on charge after all four lights are flashing, the charger goes into trickle charging mode for the final 10%. Eventually, when trickle charging is complete, and the batteries are at 100% charge, all the lights will go out. Approximate times for 100% charge are 2 hours for 2 cell (2600mAh), 4 hours for 4 cell (5200mAh) and 6 hours for 6 cell (7200mAh)