A new quantum sensor developed in Japan can measure the energy stored in electric vehicle (EV) batteries much more accurately than existing devices. The sensor uses diamond nitrogen vacancy (NV) centers and could lead to substantial improvements in EV range and energy efficiency.
Remaining energy is currently estimated by measuring the electrical current flowing from batteries as the EV is being driven. Although these currents can reach up to hundreds of amperes, their average value is typically just around 10 A. As a result, current sensors must operate over a large dynamic range, which makes them highly susceptible to noise from the surrounding environment.
This noise means that remaining energy can only be estimated to within an accuracy of around 10%. EV batteries must be recharged once they drop to 10% of their energy capacity, requiring the use of heavier batteries to achieve a target range.
Measurement accuracy is improved by use of a pair of diamond quantum sensors based on NV centers, which are formed by replacing two carbon atoms in a diamond lattice with a single nitrogen atom and an adjacent empty space. These structures behave as a tiny spin magnetic moment that is sensitive to external magnetic fields, which are measured very precisely by probing NV centers using light and microwaves.
As described in Scientific Reports, a pair of the diamond sensors were placed on either side of an EV busbar — the metal strip that connects the battery to the EV motors and other electrical components. As a current passes through the busbar, it creates a magnetic field that is measured by both sensors; one measures a positive value for the magnetic field and the other measures a negative value. Subtracting one measurement from the other eliminates noise as both measure the same levels.
Using this differential technique, the researchers measured currents in the busbar as high as 130 A and as low as 10 mA — even in noisy environments. Excellent measurement performance was also documented when the current was increased to ±1,000 A and the sensors were exposed to the -45° C to 85° C temperature range.
The researchers from the National Institutes for Quantum Science and Technology, Tokyo Institute of Technology and Yazaki Corporation suggest the sensors could reduce the weight of EV batteries by 10%.