These batteries are 300 cycle affairs so your best bet is a new battery.
I've had a Nokia Lumia 520 for (I believe?) two years. I hadn't been having any problems with the battery until this past week. All of a sudden, it is charging extremely slow, or even losing charge while plugged in and not in use. It is inconsistent because sometimes it will still charge around 30%/hr, but other times it will charge very slow (10%/hr) or discharge while plugged in. I've even been keeping it in battery saver or airplane mode constantly to try to conserve battery life.