Battery

So i have an Iphone 6s+ and i was having problems with the battery keeping its charge it would be on 100% and then run out of battery within a few mins and would not work if if was not on charge and then one night it ran out of battery and wouldnt charge up again it just shows the "needs charging" screen with the little charger and battery and then i would have it on charge for hours and it still wouldnt charge up and im not sure if i need to replace the battery or do something else.