[FJames]

Lithium-ion batteries are 3.7v, a value fixed by the basic chemistry of the cell, so any voltage exceeding that will do for charging. USB is defined to provide 5.0 volts (+/- a few %), more than enough. The more important question is how much charging current it can supply, which will govern how long the battery takes to recharge: current x time = total amount of charge delivered. A standard USB port is defined to supply up to 500ma, and USB devices are not supposed to draw more than that. USB chargers often supply more, up to 2 amps, and devices designed to use those chargers have various means of detecting when they are attached to their high-power charger instead of a computer USB port so they know they can draw more than 500ma.

However I believe that all the Kobo devices, including the Aura, are limited to 500ma max charging current. Since they do not come with their own USB charger, they are not designed to use a higher capacity, and they do not make any attempt to determine if the USB power source has a higher capacity. My Aura takes twice as long to charge as my Touch (2x bigger battery), and each takes the same amount of time to recharge from a computer USB port, or a 1 amp charger, or a 2 amp charger.

I guess Kobo figures that it's not worth the trouble to do a fast charge since the devices need charging so seldom, although the nearly 4 hours required by the Aura with its larger battery is getting up there.