Phone charging problem?

I have a Samsing Preston phone that I got in about August last year. I have never had any problems with it, but recently (in the last week or two) it hasn't been charging properly - I'll plug it in and it'll start charging, but after about 5 minutes, it'll pop up with a message saying "battery fully charged - unplug charger." -- however it hasn't fully charged - only to the second bar or so. I can start charging it again but the same thing happens. Does anyone know what is wrong/how to fix it? Or would I have to get a new phone/battery/charger?