My ipod touch 4g battery is not functioning properly

I have an ipod touch 4g. My battery hs been damaged. It gets charged to 100% in 45 minutes and gets discharged within 1hr. The battery shows dat it is fullycharged and when i use my ipod for some time, the battery drops by 30-40% but once i stop d app the battery gains 40%. I dont know if der is sm internal problem n replacing d entire ipod is nt psbl fr me . Please help me out.