This is the way I understand it. A 50W charger has a maximum output of 5 amps. A 100W charger has a maximum output of 10 amps. When charging a 4200mAh LiPo battery, the charger runs at 4.2 amps. So, a 50W charger is all that is necessary to charge the battery. The 100W charger is also going to charge the 4200mAh LiPo battery at 4.2 amps. Is there any reason to buy a 100W charger when only charging one battery?