Based on what we know, the iPad 2 comes with a 25-watt-hour (25Wh) Lithium-polymer battery. Apple says it’ll provide up to 10 hours of life on light-duty (Wi-Fi only) applications.

A simple bit of maths gives us 25Wh / 10 hrs or roughly 2.5-watts average, a bit more with 3G running (approx. 2.75-watts).

It also happens that 2.5-watts is the maximum about of power you can extract from a USB2.0 HighSpeed port (5VDC @ 500mA). On that basis, it’ll take roughly 10 hours or so to completely charge up the iPad 2′s battery – and that’s with the iPad 2 powered off.

However, with the supplied 10-watt USB power adapter, it looks as though it will be possible to charge up the iPad 2 in as little as 2.5 hours, again with the device powered off. However, this depends on the maximum current the iPad 2 can draw from the USB power brick.

Running the iPad 2 at the same time and having the screen brightness cranked all the way up will also likely slow down the charging process.

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