3. Overheating battery

The next thing will ruin your iPhone battery is overheating. This is a very important issue. If you notice, your phone will be very slow if you let it out of the sunlight too long. This is easily explicable: iPhone battery extremely hates harsh temperature. Your phone have to work 2 times harder in the high heat/cold. According to Apple, the comfortable temperature for Apple devices (iPhone, iPad, iPod, Apple Watch) is about 32 to 95 degree.