Sudden battery drops to 2%

On four occasions, in the morning having my iPhone SE on the charger all night, I will look at messages or phone calls and the phone turns completely off. When taking the phone off the charger I see that the battery is at 100%. But the phone completely dies. I have to put it back on the charger to get it to turn on. When it turns on the battery is between 2 and 10%. The phone recharges quickly, back to 100% in 20 minutes.

Does anyone have an idea what is going on?