If your battery is running out too quickly, it may be because of the number of apps you have open on your phone. A lot of apps continue to run in the background, even after you've left them. This can cause an extra strain on your battery and cause it to run out quicker than usual.
On an iPhone you can swipe up from the home screen to access the open app selection screen, (or double tap the home button) then swipe up on the apps you want to close.
For Android phones, you'll need to check with the manufacturer on how to do this.
Another common battery drain is the brightness of your screen. By dimming your screen brightness a little, you can make your battery will last a bit longer.