High-precision robust monitoring of charge/discharge current over a wide dynamic range for electric vehicle batteries using diamond quantum sensors
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-09-06 |
| Journal | Scientific Reports |
| Authors | Yuji Hatano, Jae-Won Shin, Junya Tanigawa, Yuta Shigenobu, Akimichi Nakazono |
| Institutions | Yazaki (Japan), Tokyo Institute of Technology |
| Citations | 59 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the development of a highly robust diamond quantum sensor system designed to accurately monitor electric vehicle (EV) battery current across a wide dynamic range, resolving a critical limitation in State of Charge (SOC) estimation.
- Core Achievement: Developed a diamond Nitrogen Vacancy (NV) center quantum sensor system capable of measuring current with 10 mA accuracy while maintaining a dynamic range of ± 1000 A.
- SOC Improvement: The 10 mA accuracy enables SOC estimation with 0.1% precision, eliminating the current industry standard 10% margin, thereby stretching EV driving range by 10% (e.g., 200 km to 220 km).
- Robustness: Confirmed operation across the full automotive temperature range (-40 °C to +85 °C).
- Noise Mitigation: Utilizes differential detection with two sensors placed on opposite sides of the busbar to effectively eliminate common-mode environmental magnetic field noise and excitation light noise.
- Wide Dynamic Range Control: Implements a mixed analog-digital control system for the microwave generator frequency, allowing the sensor to trace magnetic resonance over a wide dynamic range (1 GHz) without deviation.
- Environmental Impact: Achieving this precision is projected to lead to a 0.2% reduction in CO2 emissions in the 2030 Worldwide Transportation field.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Current Dynamic Range | ± 1000 | A | Maximum measurable current confirmed. |
| Current Accuracy (Theoretical) | 10 | mA | Achieved based on ODMR fluctuation noise (10 mA/Hz0.5). |
| Operating Temperature Range | -40 to +85 | °C | Full automotive temperature range confirmed. |
| Current Linearity (High Current) | ± 0.3 | % | Confirmed linearity from 40 A to 1000 A. |
| Current Linearity (Low Current) | ± 0.5 | % | Confirmed linearity from 1 A to 130 A. |
| WLTC Maximum Current Traced | 130 | A | Current supplied from battery module during WLTC pattern test. |
| WLTC Average Current | 14 | A | Typical average current during WLTC driving pattern. |
| Diamond Sensor Size | 2 x 2 x 1 | mm3 | Ib (111) crystal used for sensor head. |
| Electron Beam Irradiation Dose | 3 x 1018 | cm-2 | Used to create NV centers. |
| Annealing Temperature | 1000 | °C | Annealing time was 2 hours. |
| NV Concentration (Estimated) | 5-6 | ppm | Estimated concentration in the diamond. |
| Static Magnetic Field (Bias) | 19 | mT | Provided by Neodymium magnets to split spin states. |
| Resonance Frequency Difference (RFD) | 1050 | MHz | Corresponds to the 19 mT static field (RH - RL). |
| Gyromagnetic Ratio (Îł) | 28 | Hz/nT | Used for magnetic field conversion. |
| Microwave Generator Accuracy | 40 | ppb | Corresponds to 0.3 mA busbar current value. |
| Excitation Light Power (Per Sensor) | 100 | mW | Used for optically detected magnetic resonance (ODMR). |
| Fluorescence Collection Efficiency | ~1 | % | Current efficiency of the prototype system. |
| Lock-in Time Constant (CR) | 0.1 | s | Used in the analog control loop. |
| FM Modulation Frequency (FMOD) | 2 | kHz | Chosen to be lower than typical values (e.g., 18 kHz). |
| Current-to-RFD Ratio (at 130 A) | 1.78 | MHz/A | Conversion ratio used for SOC estimation. |
Key Methodologies
Section titled âKey MethodologiesâThe high-precision, wide-dynamic-range current monitoring system relies on three primary technical innovations: sensor fabrication, differential detection, and mixed analog-digital control.
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Diamond Sensor Fabrication:
- Material: 2 x 2 x 1 mm3 Ib (111) diamond crystals were used.
- NV Creation: Crystals were irradiated with a 3 x 1018 cm-2 electron beam.
- Annealing: Subsequent annealing was performed for 2 hours at 1000 °C to activate the NV centers (estimated concentration 5-6 ppm).
-
Busbar Current Differential Detection:
- Setup: Two identical diamond sensors (A and B) were adhered to optical fibers and placed on opposite sides of the copper busbar (2 mm thick, 20 mm wide).
- Noise Elimination (Common Mode): Sensor A measures (-Bbusbar + Bexternal) and Sensor B measures (+Bbusbar + Bexternal). Taking the difference (B - A) isolates Bbusbar while eliminating Bexternal (external magnetic field noise) as a common mode.
- Excitation Light Noise Reduction: Differential detection also reduced common-mode noise from the excitation light, halving the internal noise floor (from ~10 nT/Hz0.5 to ~5 nT/Hz0.5).
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Mixed Analog-Digital Control for Wide Dynamic Range:
- Static Bias: A 19 mT static magnetic field (from Neodymium magnets) was applied parallel to the [111] NV axis to split the resonance frequencies (RL and RH), with a difference (RFD) of 1050 MHz.
- Analog Control (Fine Tracing): A lock-in amplifier output difference (LOD) is integrated and fed back to the microwave generator to trace the resonance frequency (R) as long as the magnetic field change is within the ODMR slope width (3.5 MHz, corresponding to ~7 A).
- Digital Control (Wide Range): If the analog control exceeds a set limit (H = 3 MHz), the PC digitally adjusts the microwave generator center frequency in 100 ms cycles to keep the analog control within the slope width, enabling stable tracing over the full 1 GHz dynamic range.
- Simultaneous FM Modulation: Time division multiplexing was used to supply four FM modulated microwave frequencies (RL ± FDEV and RH ± FDEV) to the sensor in a shared timing, allowing a single lock-in amplifier to simultaneously track both RL and RH and isolate temperature drift effects.
Commercial Applications
Section titled âCommercial ApplicationsâThe high-precision, wide-dynamic-range current monitoring technology enabled by diamond quantum sensors is critical for several high-value engineering sectors:
- Electric Vehicles (EV) and Hybrid Vehicles:
- Accurate State of Charge (SOC) and State of Health (SOH) estimation, eliminating the 10% safety margin and extending driving range.
- High-precision monitoring of regenerative braking currents (up to 1000 A) and low standby currents (mA level) within the same sensor system.
- Battery Management Systems (BMS):
- Improved efficiency and longevity of large battery packs (e.g., grid storage, marine, and aerospace applications) requiring highly accurate current integration (coulomb counting).
- Industrial Power Monitoring:
- High-current monitoring in noisy industrial environments (e.g., factories, charging stations) where external magnetic interference is common.
- Quantum Sensing Technology:
- Advancement of robust, fiber-coupled, and temperature-stable NV-diamond magnetometers for real-world deployment outside of laboratory settings.
- Energy Efficiency and Regulation:
- Enabling compliance with future CO2 reduction targets in the transportation sector by maximizing battery utilization efficiency.
View Original Abstract
Abstract Accurate prediction of the remaining driving range of electric vehicles is difficult because the state-of-the-art sensors for measuring battery current are not accurate enough to estimate the state of charge. This is because the battery current of EVs can reach a maximum of several hundred amperes while the average current is only approximately 10 A, and ordinary sensors do not have an accuracy of several tens of milliamperes while maintaining a dynamic range of several hundred amperes. Therefore, the state of charge has to be estimated with an ambiguity of approximately 10%, which makes the battery usage inefficient. This study resolves this limitation by developing a diamond quantum sensor with an inherently wide dynamic range and high sensitivity for measuring the battery current. The design uses the differential detection of two sensors to eliminate in-vehicle common-mode environmental noise, and a mixed analog-digital control to trace the magnetic resonance microwave frequencies of the quantum sensor without deviation over a wide dynamic range. The prototype battery monitor was fabricated and tested. The battery module current was measured up to 130 A covering WLTC driving pattern, and the accuracy of the current sensor to estimate battery state of charge was analyzed to be 10 mA, which will lead to 0.2% CO 2 reduction emitted in the 2030 WW transportation field. Moreover, an operating temperature range of â 40 to + 85 °C and a maximum current dynamic range of ± 1000 A were confirmed.