Full battery completely depleted within a few hours without any user activity -- what to do? I left my phone on the table with full battery without doing anything with it. After a few hours the battery was completely depleted, and the phone automatically shut down.

What could cause such a thing, and what actions to take in such a situation?
Some causes and answers...


You leave a high-CPU using app/game paused or running.


Apps and Games can and do continue to devour battery if left running. If you're going to step away for awhile, make sure to exit out of any games or apps first (particularly any that usually cause the phone to get warm while using) so they don't unnecessarily become power vampires.

Cellular/Data Signal Hunting.


Especially with LTE phones, if you're in an area with bad signal the radio on the phone will continue to try to make connection and will drain the phone in doing so. Putting the phone in Airplane mode before leaving it for extended periods will stop the radio from 'hunting'.

Too many background activities (or updates set too often).


Twitter, Facebook, Email, etc... Setting how often they communicate with their servers (like switching from every 5 minutes to once an hour) can make a big impact on battery savings.