Why doesn't my iPhone fully charge?

When my iPhone 4 runs out of battery it charges until around 10% and then it stops. Yesterday it fully charged and I don't know how it did. But the charger is fine because my iPod charges fine with it. So what's the solution? I don't want to throw the phone away and I don't want to take it into repair unless I really have to. So how can I get my iPhone to charge? It also won't connect to my PC.