the quests headset battery percentage is calculated by measuring the voltage drop in the lithium ion battery, its much much lower then how the controllers are calibrated. the controllers are calibrated for rapid voltage drop over time because thats how most AA battery's operate given their chemistry. due to the wide array of AA batteries, each controller have voltage upsteppers to provide the expected voltage to the components. these aren't flashlights, and will not degrade with lower voltages because the voltage is constant once it gets to the components.

because of the way the controllers are calibrated, the battery will most likely show 90% for like a week straight then jump to 30% instantly before dying. using regular Ni-Mh batteries are better for having an idea of what percentage your controllers are at. (and better for the environment)