I have had a similar issue so i thought i should speak up.
yesterday around noon i drained my battery to 20% playing six guns,once it hit 20 the phone shut off and when plugged to a usb will give me the battery icon,when plugged into the wall it goes straight into a boot loop.
i even have a charger that allows for the battery to be charged outside of the phone. but that doesnt work, nothing has so far.
here are two things that i have acknowledged to take into consideration for practicing good battery health.
try to keep the battery from dying as much as you can
the charger i have been using is a 5v charger,the nokia lumia 521 has a 3.7v battery (people obliviously overcharge their battery and destroy the cell like me)
ALWAYS try to stick with the charger that is designed for that phone,i wouldnt try hard resets until you got a new battery and train it (charge to 100% before using the battery)