Precise Current Monitoring over a Wide Dynamic Range for Electric Vehicle Batteries Using Diamond Quantum Sensors

Diamond quantum sensors with NV centers potentially have high sensitivity and a wide dynamic range. This feature is advantageous for EV battery monitoring applications. The peak value of EV battery current reaches ± several hundred amperes. So far, the accuracy of commercially available current sensors with a measurement range of ± several hundred amperes is about 1 A. On the other hand, the average battery current over the driving hours is about 10 A. Therefore, about a 10% margin is required in the remaining cruising mileage estimation. If the accuracy of the current sensor can be improved to 10 mA while maintaining a maximum range of ± several hundred amperes, the state of charge (SOC) of the EV battery can be accurately determined and the EV's cruising mileage can be extended by 10%.<br/><br/>In our prototype battery current monitor, a diamond sensor bonded to the end of an optical fiber was attached close to a copper busbar of 20 mm width and 8 mm thickness. The diamond sensor is a 2 × 2 × 1 mm³ type Ib (111) crystal treated by 3 × 10¹⁸ cm^-2 EB irradiation and 1000°C annealing for 2 hours. The sensor contains 5-6 ppm NV- centers. A two-tone microwave is applied to the diamond sensor through a copper tape adhered to the sensor. The change in the fluorescence in response to the microwave frequency is the Optically Detected Magnetic Resonance (ODMR) spectrum.<br/><br/>The magnetic field is measured as the resonance frequency difference, that is, the difference between the low and high resonance frequencies in the ODMR. The (111) plane of the diamond sensor is placed perpendicular to the busbar surface as well as parallel to the current direction. The magnetic field at the diamond sensor position caused by the 10 mA busbar current is 170 nT. The sensitivity of 5 nT/√Hz enables the 10 mA detection in 1 kHz bandwidth. Three key points to ensure accuracy is as follows:<br/><br/>(1) Differential measurement<br/>Differential measurement using a pair of diamond sensors placed above and below the busbar allows excluding external noise as common mode. Another purpose of the differential measurement is the elimination of the static magnetic field fluctuation caused by the temperature change. Two neodymium magnets are placed several centimeters apart from each other to provide about 20 mT static magnetic field at the diamond sensor so that the low and high resonance frequencies are separated even at the busbar current of ± several hundred amperes. Two diamond sensors are placed at the midpoint between these magnets, symmetrically positioned vertically across the busbar, so that the static magnetic fields applied to the two sensors are equal. This also eliminates the effect of the static magnetic field change due to the temperature change as common mode.<br/><br/>(2) Stable tracking of resonance frequency<br/>The narrowness of the ODMR linewidth enables to follow the resonance frequency sharply by analog feedback from the photodiode output to the frequency modulation input of the microwave oscillator. On the other hand, the feedback is stable only within the linewidth. Therefore, the carrier frequency of the microwave oscillator should be intermittently adjusted so that the FM modulation amount be kept within the linewidth.<br/><br/>(3) Suppression of transverse magnetic field<br/>To keep precise linearity between the resonance frequency difference and the busbar current, the transverse magnetic field applied to the [111] NV-axis by the busbar current should be minimized. For that purpose, the (111) plane should be carefully aligned perpendicular to the busbar surface as well as parallel to the current direction.<br/><br/>This work was supported by the MEXT Q-LEAP (JPMXS0118067395).