Iphone Battery Indicator is not good at indicating percent of battery left

The battery on work iphone 6S this morning:

10 am: 56% and not charging even though it was plugged in over night in the smart battery case.

10:45: 56% and charging after I took off the smart battery case to charge separately.

11:15: 56% the phone says, “I’m turning off now because I’m tired.”

11:22: after restarting 5 times 17% and charging but who knows if thats true?