Diamond quantum sensors used to extend EV driving range

A diamond quantum sensor prototype developed in the MEXT Q-LEAP Flagship project by researchers from the Tokyo Institute of Technology (Tokyo Tech) and Yazaki Corporation could resolve the issue of battery usage inefficiency in electric vehicles, resulting from an inaccurate battery charge measurement. The sensor can measure currents in a wide range as well as detect milliampere-level currents in a noisy environment, improving the detection accuracy from 10% to within 1%.

The popularity of electric vehicles as an environmentally friendly alternative to conventional gasoline vehicles has prompted researchers to develop high-efficiency EV batteries. However, a major inefficiency in EVs results from inaccurate estimations of the battery charge. The charge state of an EV battery is measured based on the current output of the battery. This provides an estimate of the remaining driving range of the vehicles. The battery currents in EVs can reach hundreds of amperes. However, commercial sensors that can detect such currents cannot measure small changes in the current at milliampere levels. This leads to an ambiguity of around 10% in the battery charge estimation; as a result, the driving range of EVs could be extended by 10%, which could reduce inefficient battery usage.

A team of researchers from Japan, led by Professor Mutsuko Hatano from Tokyo Tech, have devised a solution. In their study published in Scientific Reports, the team reported a diamond quantum sensor-based detection technique that can estimate the battery charge within 1% accuracy while measuring high currents typical of EVs. “We developed diamond sensors that are sensitive to milliampere currents and compact enough to be implemented in automobiles. Furthermore, we measured currents in a wide range as well as detected milliampere-level currents in a noisy environment,” Hatano said.

The researchers made a prototype sensor using two diamond quantum sensors that were placed on either side of the busbar (electrical junction for incoming and outgoing currents) in the car. They then used a technique called ‘differential detection’ to eliminate the common noise detected by both the sensors and retain only the actual signal. This enabled them to detect a small currently of 10 mA amid background environmental noise. The team then used a mixed analog-digital control of the frequencies generated by two microwave generators to trace the magnetic resonance frequencies of the quantum sensor over a bandwidth of 1 GHz. This allowed for a large dynamic range (ratio of largest to smallest current detected of ±1000 A. A wide operating temperature range of −40 to +85°C was confirmed to cover general vehicular applications.

The researchers tested this prototype for Worldwide Harmonised Light Vehicles Test Cycle (WLTC) driving, a standard test for energy consumption in EVs. The sensor accurately traced the charge/discharge current from -50 to 130 A, thereby demonstrating a battery charge estimation accuracy within 1%.

“Increasing battery usage efficiency by 10% would reduce battery weight by 10%, which will reduce 3.5% running energy and 5% production energy of 20 million new EVs in 2030 WW. This, in turn, corresponds to a 0.2% reduction in CO² emissions in 2030 WW transportation field,” Hatano said.

Image credit: iStock.com/Fritz Jorgensen