iPhone Battery Troubleshooting
Batteries are the life source of any mobile electronic device. When something isn't working as expected, check the battery.
If you're performing a repair and notice afterwards that your battery is running at a higher than normal temperature this could indicate you accidentally damaged something. When your battery is heated up it's because the CPU on the logic board is running at 100% at 100% of the time! Think of the CPU like the engine of your car. Cars use the most energy when you're accelerating. When you step on the gas, your car revs up until you achieve a cruising speed and let off the pedal. It takes a lot more work for a car to accelerate than it does to cruise. In your car, you don't keep the pedal to the floor all the time, it wastes gas! Not only that it forces the engine to operate at maximum capacity all the time. That's exactly what is happening with your iPhone.
The CPU is so efficient that it uses less than 5% of it's capacity on average. In most cases when your iPhone is operating at a higher temperature it means that your CPU is working double time. And in 99% of cases, that means you have a software problem.