Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
There is a problem with the algorithm used in older versions of the firmware when calculating remaining battery life.
Ensure you are using the latest version of firmware on your D750:
Your satisfaction is my priority; Kindly take a moment to rate my service 5 stars at the top of your question page so I am rewarded for my time; tips appreciated :)You can always reply back at any time if you need any further assistance at all.Pete.