Battery goes from 50% to dead instantly? Update 7 is draining battery

First in the general sense, the radio portion of a phone can and often does use the most power from the battery. To save on battery life, turn off location services on all apps that don't absolutely need it (find my phone for example needs it, don't turn that off). In addition, turn off background app updates. If you're not looking at an app, there no need to let it update over the radio. Look around at the settings and make intelligent selections for how you use your phone and you'll find the battery can last very long.

Specifically, your phone going from 50% down to 0% suddenly sounds like a hardware problem to me. That should not happen.

reset all settings

settings-general-reset-reset all settings

now reconnect to wifi

settings- wifi- click network name- enter password - join

if issue persist back up and restore as new via iTunes

Peace, Clyde

I would follow advice provided by Clyde. Your battery counter is simply off and he provided you with correct ways to set it up correctly. The only change that I would make before going to extreme "restoring in itunes", I would also try to completely discharge and then completely charge phone to 100 percent - it usually does tend to fix a problem if reset of all settings didn't.

See this link provided by another user http://osxdaily.com/2013/09/19/ios-7-battery-life-fix/