Why does my iPhone run out of battery so fast?

I recently purchased a iPhone. Well I took over his contract. Anyways when I first for had it my battery life was amazing by the end of the day I'd still have 40% maybe even more. But now it seems to run out very fast. I have co op on the afternoon till 530 so I don't touch my phone when it's 530 my battery would be 70% when before it would be 80% I'm most def sure I don't have apps running. But can anyone please walk me through and tell me why my battery is drained? I'm new to apple so I'm a newbie. Any help would be greatly appreciated!

You must've exceed the 200 charge cycles, consult geek squad or another service to change the battery.

Apple product run about of battery fast if they are used a lot, and if they are not new, battery tends to drain faster the longer you have it.

Turning down your brightness helps a lot, but overall it's just iPhones I have one and I charge it every night… By the end of the day I have about 20% left… They also sale iPhone cases that have a charger built into it that you flip the switch and it keeps it from dying… Not sure how good those work but a lot of people have them… It also depends what your Doing while your on it… If you use the Internet or stream music off of pandora or YouTube it kills the battery… Listening to music bought from iTunes barely affects the battery and taking pictures barely affects it… Good luck!