• Use your iPhoneuntil it shuts down on its own. Turn on the flashlight, turn up the screen brightness all the way, and play a video, preferably one that is streaming from the Internet, if the battery life is approaching 0% and you want to drain it faster.
• Allow your iPhone to sit overnight to further drain the battery.
• Connect your iPhone to the charger and wait for it to charge. Make sure you're using the Apple charger or one with the same power and amperage.
• Swipe 'slide to power off' while holding down the sleep/wake button.
• Allow at least 3 hours for your iPhone to charge. Older iPhones require a 5-hour charge. While your iPhone is turned off, the charge progress indicator is not visible
• To start up your iPhone, press the sleep/wake button for roughly a second while the charging wire is still connected.
•  Once the iPhone has booted up, press and hold the sleep/wake and home buttons until the Apple logo appears. Hold the volume up and the sleep/wake button together if you have a newer iPhone without a physical Home button.

Read More: What are the signs that you have hardware problems in iPhone?