Some 3rd party (and sometimes even 1st party) applications for your smartphone are not properly created and they may prevent your phone from going to standby/sleep mode when you lock it’s screen. This can result in a very short battery time leaving you with a dead battery in the middle of the day.

There is an easy way to test each application for this “juice leak”:

On Android devices, somewhere in Battery Settings app (varies with Android version) you can see the system Up time and Awake time. The stock phone should go to sleep soon after you lock it’s screen. Run your application and lock the phone. Leave it for at least a minute, then unlock and go back to the above screen to see if the Awake time has increased. If yes, then it’s time to get rid of it and find another one.