Q: My iphone keeps running out of battery really quickly.  Instead of a green battery icon when I charge it, I get a red line.  It charges fine, but if it's a 100% when I go to bed, it's empty the next morning.  What should I do?

Surely an hardware problem. If it's under warranty, take it back

A battery below 10% will be red when you first begin charging it. It will turn green at about 20%.. But if the battery goes dead as quickly as you say there is definitely something wrong. Make an appointment at an Apple Store and have them look at your device.

make sure to close down all running apps at the background of your phone before you go to sleep. double press the home button adn close all running apps then observe. if issue persists. have it diagnosed on the apple store.

violetsmith wrote:

 

... has a red line on the battery icon when I charge it instead of a green one.  ...

It stays RED until it reaches 20%... then it goes GREEN all the way to 100%.

violetsmith wrote:

 

My iphone keeps running out of battery really quickly. ...

Set it Up as a New Device

http://support.apple.com/kb/HT4137

Also consider Deleting any Apps you have Purchased / Downloaded but you now never use..