Before getting an iPhone 4 I upgraded my 3G to IOS 4 and used it for several weeks and noticed that sometimes when charging it, the battery indicator didn’t update, even after a couple of hours or all night. This seems to happen when charging from the mains.

It’s either a bug with the IOS 4 software particular to 3G or there’s something not right between chargers.

How to Correct the Indicator
(1) Reset the iPhone. i.e. hold both buttons down until the “slide to power off” swipe appears and then switch it off and restart.

or

(2) Plug it into a Computer with iTunes and charge it.