Messed up battery

[Silver15]

So a few days ago, my battery was fine. Wouldn't really drain at all when not in use. For the past 2 days it has been acting up, though.In about 2 hours, it went from fully charged to 49% battery life while just sitting on my desk, idle. Then I went to do some stuff to save the battery such as killing apps, removing live wallpaper (been using it 2 years, didn't have problems with it draining life), etc. Just some simple stuff. In the 5 minutes that I did this on my phone, the battery went from 49% to 41% which has me worried now. I removed the battery just to see if the interior of the phone was damaged or something, but saw that it looked just as it has for the past 2 years since I got it. I put the battery back in after taking it out at 41%. It tells me my battery life is at 6% now just after taking it out for 1 minute from 41%...do I need a new battery? Something is clearly wrong with my phone. The battery won't last long if it is like this. What should I do?