I picked up a really beat iPhone to tinker with. The LCD was completely destroyed, as in, nothing left of the LCD but the backlight panels. Touchscreen and display were physically torn out. The backlight would turn on with a press of the power button and would also turn on when plugged into USB which was a good sign, although my PC couldn't see it.

I put a spare LCD in the iPhone and on battery, it is completely dead. No life whatsoever. If I plug it in, it boots up to the Apple logo, then shuts off and reboots. It repeats this several times before quitting. It will repeat this process if I press the sleep/wake button. If I press and hold sleep/wake and Menu for 10-seconds to reset it, it will go into Restore Mode. However, it's completely dead when plugged into USB now, and only tries to boot when plugged into the AC adapter. The battery voltage was only 3.3V and should be 3.7V, although I left it overnight and it didn't try to charge the battery. The solder connections look fine, and I did not desolder it when changing the LCD. If I leave the iPhone in restore mode very long, it gets quite warm.

Does this problem sound familiar to anyone? What could the problem be? It almost sounds like the battery is too dead to boot, and USB being limited to only 500mA is not enough to power it which is why it only tries to boot on the 1A charger.

I'm almost certain I didn't short out anything when reinstalling the motherboard since I used plastic tools.