Re: Bb 8520
If the battery charges ok and works ok in another device but your device runs the battery down fast for no apparent reason then then the device is probably faulty. I would still try another battery, one you know for certain is good. If you can do a clean install of the OS, then do it. Test before you restore and reload apps. If you still have a problem, and you are using a battery you know is good, then you may conclude the device is broken.
Personally, I would get a new device, or at least an OS7 device.