Helping to neutralize carbon Diamond quantum sensors monitor electric vehicle batteries with high precision
What can be done when battery power is not measured correctly and battery usage is inefficient during the use of electric vehicles?
This problem will be solved. In Japan's Q-LEAP flagship project, a team from Tokyo University of Science and Technology and Yazaki Corporation have jointly developed a new diamond quantum sensor [1] - which can measure a wide range of currents and also detect milliamp currents in noisy environments, with improved detection accuracy from 10% to within 1%.
01 A long-standing problem: Electric vehicles struggle to accurately estimate power
As an environmentally friendly alternative to conventional gasoline vehicles, the popularity of electric vehicles (EVs) has been on the rise. As a result, research efforts to develop efficient EV batteries have flourished. However, one of the major inefficiencies of EVs is: inaccurate estimation of battery power. The state of charge of an EV battery is measured based on the battery's current output, which also provides an estimate of the vehicle's remaining driving range.
Typically, battery currents in electric vehicles can reach hundreds of amps. However, commercial sensors capable of detecting such currents cannot measure small variations in milliampere current: this results in an ambiguity of about 10% in the battery charge estimate. This means that typically electric vehicles can travel up to 10% longer, which also leads to inefficient battery use.
02Diamond quantum sensor: Power detection accuracy of 1%
A Japanese research group led by Professor Mutsuko Hatano of the Tokyo Institute of Technology (Tokyo Tech) has now come up with a solution. In research published in Scientific Reports [2], the team reports a detection technique based on a diamond quantum sensor that can estimate battery power within 1% accuracy even when measuring high currents in electric vehicles.
"We developed a diamond sensor that is sensitive to milliamp currents. Its compact structure allows it to be implemented in vehicles; in addition, we measured a wide range of currents and detected milliamp levels in noisy environments." Professor Hatano explained.
In their work, the team made a prototype sensor using two diamond quantum sensors that were placed on either side of a bus bar (the electrical connection point for input and output currents) in the car. They then used a technique called "differential detection" to eliminate the common noise detected by the two sensors, leaving only the actual signal: this in turn allowed them to detect small currents of 10 mA in the background ambient noise.
Next, the team used a hybrid analog-digital control of the frequencies generated by the two microwave generators to track the magnetic resonance frequencies of the quantum sensors over a bandwidth of 1 gigahertz. This resulted in a large dynamic range (ratio of maximum current detected to minimum current) of ±1000 A; in addition, the operating temperature range was as wide as -40 to +85°C, covering the temperature difference of typical vehicle applications.
Finally, the team tested the sensor prototype in the Worldwide Harmonized Light Vehicle Test Cycle (WLTC) driving, a standard test for electric vehicle energy consumption. The sensor accurately tracked charge/discharge currents from -50A to 130A and demonstrated accuracy in battery charge estimation to within 1%.
(a)Battery current sensor usage in EV. The current from a battery module with stacked battery cells passes through the bus bar in the junction box and is measured by the current sensor. (b) Driving speed (km/h) and (c) conversion current in WLTC (Worldwide Harmonized Light Vehicle Test Cycle) mode 1. A typical car is assumed to weigh 1500 kg. The maximum current value is 126 A and the average is 14 A. (d) Effect of a high-precision current sensor on extending the EV driving range. A 10% margin is required to estimate the battery state of charge based on a current sensor with an accuracy of 1 A. A sensor with an accuracy of 0.01 A eliminates this margin.
Prototype differential detection system. (a) Diamond sensors are adhered to one end of the fiber. (b) Sensors A and B are placed on both sides of the busbar for differential detection. (c) Photograph of the sensor and busbar placed between a pair of magnets. (d) The ODMR spectrum of the diamond sensor. (e)Block diagram of the busbar current measurement system. (f) Time-division microwave frequency modulation of the lock-in amplifier to reflect the magnetic field as its differential output.
Current measurement results. (a) Input current waveforms of ±1000 A in 2 s steps of 20 A. (b) Observed resonant frequency difference (RFD), with all waveforms correctly tracking the bus current. (c) Ratio of the average of the resonant frequency differences over the sampling window in each step versus the bus current. A linearity of ±0.3% C was confirmed over the current range of 40 to 1000 A and the temperature range of -40 to +25 °C, and over the current range of 40 to 500 A and the temperature range of +4 to +85 °C. (d) Fluctuation of the resonant frequency difference within the sampling window at each step.
03Battery efficiency improvement → Weight reduction → Helps carbon neutrality
This experiment developed a prototype battery monitor using a diamond quantum sensor that can estimate the battery charge status and accurately predict the remaining driving range of electric vehicles.
What are the implications of these findings?
Professor Hatano said, "Increasing battery usage efficiency by 10% will result in a 10% reduction in battery weight, which will result in 3.5% less operating energy and 5% less production energy for 20 million new electric vehicles in 2030; this in turn is equivalent to a 0.2% reduction in CO2 emissions in 2030 in the transportation sector."
Reference links：
[1]https://www.titech.ac.jp/english/news/2022/064791?utm_source=nationaltribune&utm_medium=nationaltribune&utm_campaign=news
[2]https://www.nature.com/articles/s41598-022-18106-x
