Why is my iPhone 4 shutting down with 20% battery left?

For the past week, my phone's been shutting off when it gets to 20% battery life. I don't know why, as I make sure background apps are shut off…
Anyone know any solutions to stop this happening?

Thank you!

somethin's wrong with your battery

Hi Sarcastica
I suggest trying the remove battery for 30 seconds then reinstall battery, power on and charge to full and see if it helps Good luck!