The iPhone 3G has a built-in 3.7 volt rechargeable lithium-ion battery, rated at 1150 mAh. This battery requires a charging voltage of 5 volts, which can be acquired from a variety of sources including USB sockets and car cigarette lighter adaptors. It's very important that the iPhone receives the correct charging voltage at a suitable current supply.
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The iPhone 3G's 3.7-volt internal battery requires a 5-volt input supply to charge. This is a very common charging voltage with electronic devices, as all computer USB sockets output a stabilised, regulated 5-volt supply. Therefore, any USB socket is able to provide the correct charging voltage for an iPhone 3G, though some lack the required current to charge the device from flat.
The iPhone 3G requires a minimum charging current of 500 milliamps-per-hour (mAh), which is the standard current supply of a computer's USB socket. This will charge the iPhone 3G quite slowly and will often fail to charge a device that is completely flat. A minimum current of 1 amp is preferable, as this will ensure quick, complete charging of the iPhone 3G regardless of its battery level when charging begins.
In order for the iPhone 3G to accept a supply from a charger, it must be officially approved by Apple. Computers and USB sockets on peripherals such as printers are accepted since these are regulated at the required 5 volt supply. Many third-party chargers and USB adaptors don't have Apple's licensed hardware identification built into them, and will be prohibited from charging by the iPhone's operating system. Always ensure the charger is verified as iPhone-compatible before purchasing.
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