The likely source of your problem could be that the phone's battery is loose. Try adding a folded piece of paper on the bottom of the battery, so that the contacts on top are pushed into the battery. Make sure that the battery can stay in place.
If this doesn't work, the problem could be the battery itself is bad. You'll have to replace it if this is the case.
Check out [[Nokia Lumia 521 Troubleshooting|Nokia Lumia troubleshooting page.]]