S4 battery suddenly flat.

[MrTedd]

I can be using my phone on the internet with almost full charge, without it being plugged in and suddenly the phone will switch off.. it then continuously tries to reboot and fails, and reboots and fails... When I then plug it into the mains charger it boots up and shows a flat battery.. which it then charges.

Any ideas please??? would a factory reset help?

Thanks in advance
M.

 

[MrTedd]

Also.. if I switch my phone off and take the battery out, then boot it up without the charger in.. its then fine as before, the battery shows its normal charge.