iPhone 3GS endless reboot after battery replacement fix
With a lot of iPhone 3GS units now requiring replacement batteries, but with no original batteries available, sometimes after a replacement when the power percentage drops to 30~50% the iPhone will turn off, but when you try to turn it back on again it goes in to an endless cycle of showing the Low-Battery screen and then attempting to start with the Apple logo, then restarting all over again in a minute or two.
The exact cause of the problem isn't known, though a mismatch between the presumed or old battery charge curve and the new battery is likely. Mostly the iOS installed thinks that the battery still has 30~50% of power left when in fact it empty, a lot of this is because lithium cells have an exceptionally flat discharge curve and thus estimating the energy left is not always practical without keeping track of the mAh consumed.
- Put the iPhone 3GS into DFU/Recovery mode
- Hold down the power button for 4 seconds
- Hold down the home button for 10 seconds
- Release the power button
- Wait 15 seconds
- Release the home button
- Charge for 30 minutes
- Exit from recovery mode
- Press the home + power button for 15 seconds
- Release both buttons
- Press the home button for 3 seconds
- The phone should now be booting into normal mode and you should be able to continue charging normally once more.