https://www.scientificamerican.com/article/ultrasensitive-fuel-gauges-could-improve-electric-vehicle-batteries/

A team of U.S. and German scientists is developing detectors that could remove some of the mysteries—and hazards—of using large lithium-ion batteries to power vehicles and store renewable energy.

The solution they see will rely on ultrasensitive new detectors, specialized magnetometers that can produce an image of the magnetic field of a given battery, revealing whether they have defects and showing exactly how much electric charge is left.