[iMore Question]

I was using the phone and saw that it shows 59% when it suddenly black out. Last night i put it to charge and it shows 29% after 8 hours on charge. I turned it off and on and it then shows 100%. Do you think it is a problem with the phone or the battery? Thanks!

[Just_Me_D]

Not sure. If it were me, I'd restore the device and set it up as new and then see if it happens again. If not, great. If so, make a Genius Bar appointment with the nearest Apple Store to have them diagnose the problem for you.

[Ledsteplin]

Let it run down to 1%. Then charge it up to 100%, allowing it to continue to charge about 30 minutes after reaching 100%.
Also, if you want to test the battery, try this test.
Write down your usage and standby time, press the sleep/wake button (or lock button, as some call it) to put the device to sleep, and set the device down for five minutes. When you come back, take note of the change in time. If your device is sleeping properly, then the Standby time should have increased by five minutes and your Usage time by <1 minute 3. If your Usage time rises by more than one minute, you have a drain problem. Something is keeping your device from sleeping properly, significantly shortening the time it will last.