iPhone 4s 2 years old. Battery not holding.

I got given my iPhone last November 2013 but second hand. And I've charged it up till 100% but if I'm going out and it's below 50 or 50 i will charge it. And I got told off a friend try not to use it as much and just to let it die and charge it afterwards. I have tried to regain my battery but it's not worked. Would I need to replace my battery? Any suggestions?

Thank you