LogFAQs > #979768240

It depends a lot on the battery and the charger. Most newer batteries and fast chargers now use higher voltage for the initial "fast" part of the charge, as much as 48V. Watts = Volts x Amps. More amperage means more resistance, which means more heat and more damage to the battery. Therefore, using a higher voltage allows for more wattage with lower amps. 15W at 5V is 3A, but at 9V it's only 1.67A.

Most newer chargers are designed to do an initial high voltage fast charge, then they ramp down the voltage to top off the battery at a slower rate. You will actually see almost no difference between a 10W charger and a 45W charger for the final 90 to 100%, and little difference between a 45W and 25W chargers for going from 50 to 100%. But going 0 to 50% it makes a HUGE difference.