Tests show that the iPad 2 battery (25 watt-hours) will charge to 90% in 3 hours 15 minute. It will charge to 100% in 4 hours 15 minutes.
The new iPad 3 has a larger capacity battery (42 watt-hours), so using the 10W charger will obviously take longer at up to 6 hours. If you are using your iPad while charging, it will take even longer. It's best to turn your iPad OFF and charge over night if possible
The rate of charge will vary during the charge from 10 watts to start with down to 2 watts at 95%
The output for the iPhone charger or some compatible chargers such as the Amazon Basics is 5 watts. These will charge but take twice as long.
Always use the charger that came with the iPad and use genuine Apple products and you shouldnt have any problems. Using 3rd party products can cause lots of problems and also void your warranty.
A lot of so called iPad charger cables retailing for a couple of Pounds and badly made have a high resistance and may deliver up to about 3 Watts or less sometimes giving a warning message that it is not charging or not compatible.
The iPad 4 has a new charger with 12 watt output and a different connector at the iPad called a Lightening connector. This charger is compatible with older models using the original USB to 30 pin connector.
Data for the iPad 2 charger which is marked nominally as 10 watts is as follows from measurements taken and calculated. ( V=IR. & watts=volts x amps)
Volts= 5.1 DC
Ohms = 2.43
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