You might try running a battery test. My first Fire 8.9 did very strange battery things. It routinely shut down due to low battery while showing between 20% and 40% charge. I turned wifi off and played a previously downloaded video continuously and couldn't get more than about 5 hours on a full charge. Customer service confirmed I should be getting better battery life.
I exchanged it and the new unit has no battery quirks. It can play video with wifi off for more than 8 hours. It actually drains all the way down - no low battery shut offs while showing high percentages.