That can't be the explanation. Batteries use fewer watts as they get close to full.

That's not fully true, and even if it's partially true in some cases (this depends on the chemistry of the battery): volts and watts aren't the same thing. You can be fully capable of supplying [email protected] and not capable of supplying 12v@1A which are the same number of watts.

Battery tech is a horrible black hole that is not very fun to dig into, chargers are a little bit more transparent: with markings for various voltages and amperages printed on the device.

iPad batteries output 3.7v if I'm not mistaken, but I'm unsure what they charge with.

  > iPad batteries output 3.7v if I'm not mistaken, but I'm unsure what they charge with.

For those not familiar with the tech, the term "3.7v battery" means that it is about 4.2 volts when full. Black hole indeed.

I would expect much bigger issues and failure to charge at all if there's not a reasonable voltage on the USB line.

A 3.7V nominal li-ion battery would peak at about 4.5V while charging. A bit high, but a well designed circuit should be able to do that off 5V. Besides, 75% is far short of where the voltage starts to spike.

>volts and watts aren't the same thing. You can be fully capable of supplying [email protected] and not capable of supplying 12v@1A which are the same number of watts.

For the layman, the equation is Volts x Amperes = Watts.

Where if we use the common water examples: Voltage is electric charge ("water pressure" or "volume of water"), Amperage is electric current ("water flow rate"), and Wattage is electrical energy ("amount of water transferred").

2V x 6A, 4V x 3A, 1V x 12A, 12V x 1A and similar are all 12W but they are obviously very different in nature.

More … peak voltage or something like that?