As is the case with other variants of the device, Apple's iPhone 3G is powered by a built-in lithium ion battery, one you charge either from a wall outlet or from your computer to replenish when it runs out. If your iPhone 3G doesn't hold a charge, several causes could be to blame.
One cause of premature or quick iPhone battery draining is partially charged batteries. The iPhone 3G's operating system allows you to see the specific percentage your battery has charged, which allows you to better gauge when it's OK to disconnect the device from your charger. Ideally, you want its battery to be as close to 100 per cent charge as possible. To enable this option, tap "Settings," then "General," and then tap the grey "Off" button next to "Battery Percentage" to change it to a blue "On" button.
Computer vs. Wall Charging
iPhones that charge properly tend not to lose a charge as easily. Two options for charging the iPhone 3G exist. You can use a wall adaptor and plug the device into a wall outlet. You can also plug it directly into one of your computer's USB ports. If your iPhone 3G doesn't hold a charge after plugging it into a wall outlet, try plugging it into a different outlet. It's possible a problem exists with the outlet you typically use. Likewise, try using different USB ports to charge or -- if all of them seem to be defective -- charge in a wall outlet rather than use your computer.
Faulty iPhone USB Cable
Both charging options require you to use an iPhone USB cable, so if neither charges the iPhone adequately -- and this results in premature battery draining -- the USB cable itself could be to blame. Replace your USB cable either by purchasing a new one directly from Apple, or by visiting your local electronics store and purchasing a generic cable for any of the third-party manufacturers who sell them.
Failure to Lock Screen
The iPhone 3G's failure to hold a charge could also be that it runs applications without your knowledge -- namely, if you fail to lock the screen before placing it in storage. For example, an unlocked iPhone 3G moving around in your pocket could inadvertently begin playing songs on the device's built-in iPod media player, which eventually depletes its battery. To avoid this from happening, always click the "Lock" button on top of the iPhone before you store it.
Old or Defective Battery
If charging or screen lock issues don't explain why your iPhone 3G doesn't hold a charge, its battery may simply by old and need to be replaced. Apple begin offering the iPhone 3G for sale in mid 2008 -- and no longer sells new devices -- so it is conceivable that any iPhone 3G battery could be old enough to need replacement. The iPhone warranty doesn't generally cover batteries. Expect to pay out of pocket for the replacement, whether you purchase it from Apple or a third party.
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