iPhone Battery

My iPhone keeps quickly running out of battery and turns off.

Its is 100% charged and then quickly drops to 53% then 31% to 10% and turns off. You can then turn on and it opens with 31% battery and the back to 10% and turns off again.

It will only stop on and operate when plugged into a charging device. It worked find until I accepted and iPhone settings change & T&Cs and has not worked properly since. I have don't eh iO15 up-date but is still not working correctly.