phone keeps rebooting saying battery drained shutting down

my phone was just moving from 8 or 15% to 35% if you keep removing and placing back he charger then it shoots up to 85%, similarly it will just go down drastically. it then switched off when i plug it in on the charger it switches on and after booting just as it gets to the home screen it immediately says battery drained shutting down, then continues to do this on and on in an endless cycle connected to the charger, removing the charger it just goes off.