A friend gave me his old iPhone 4 CDMA to try to fix. It was horribly cracked on both sides, so I replaced the front display and back panel. Everything works as it should but there is still one problem: It only turns on when plugged into a wall charger. As soon as I remove it from power, it shuts off. At first, I suspected it was the battery, but that is not the problem because I swapped my iPhone's battery with the broken iPhone - my iPhone worked with his battery yet his didn't work with mine. I've looked around online and can't seem to find any articles specifically addressing this issue with an answer to it. Does anyone know what part is damaged or how to fix this issue? Any help is appreciated - thanks in advance!