The average battery in a cell phone is good for 900-1000 charges. That sounds like a lot, but it really isn't. That's less than 3 years of charging it every day, and that doesn't even mean a full empty-to-full recharge. This is the nature of lithium ion batteries of the size you typically find in most cell phones, apple, samsung, lg, htc, etc. It's not a phone thing, it's a lithium thing.
Anyway, the whole 5 series is pretty old as cell phones go, so... yeah, this isn't surprising. :\