Paul van Gerven
28 September

By measuring the current outputs of battery packs much more accurately, a Japanese quantum sensor could extend the driving range of electric vehicles by as much as 10 percent.

Not knowing exactly how much juice is left in your electric vehicle’s battery pack isn’t merely anxiety inducing, it limits range. After all, if the onboard sensors can’t give you an exact range, you better err on the side of caution.

This is a very real problem even for modern electric vehicles (EVs). The charge state of an EV battery is measured based on the current output of the battery, which in turn is used to estimate the vehicle’s remaining driving range. At the moment, a 10 percent margin is necessary because commercially available current sensors aren’t accurate enough: about 1 ampere is state-of-the-art and the average battery-current output is 10 amperes.

If you’re wondering why there’s not a better sensor available in the 21th century, keep in mind that the sensor has to deal with a wide range of currents. When the pedal is put to the metal, the output can reach several hundreds of amperes.

Harnessing the power of quantum technology, researchers from Tokyo Institute of Technology have developed a sensor that measures current more accurately over a wide range. And despite what you might expect from a quantum device, it works just fine in a noisy environment such as an automobile.

Complicated

The Japanese quantum sensor is based on so-called nitrogen-vacancy (NV) centers in diamond. These point defects are formed when a nitrogen atom and an adjacent vacancy substitute for two carbon atoms in the diamond lattice. When all’s said and done, this leaves one unbound pair of electrons in an environment that’s shielded enough that its different quantum states – particularly its spin state – are fairly stable even at room temperature or above. That’s one reason why NV centers make for interesting sensor platforms.

Another reason is that NVs are photoluminescent: when hit with green light, they start emitting red light. Since this response is altered by external factors such as a magnetic field in a very predictable way, optical measurements provide a convenient method of quantifying the external factor responsible for the changed photoluminescent response. In practice, it’s a little bit more complicated, as microwaves are also involved to probe the NV. This technique is called optically detected magnetic resonance, and it’s extremely sensitive.

Carbon dioxide

The Japanese researchers constructed a prototype sensor using two diamond quantum sensors that were placed on either side of the busbar, ie the conductive bar through which all incoming and outgoing current flows. This setup allows for differential detection, eliminating noise detected by both sensors and leaving (mostly) the actual signal.

The results are impressive. During a ‘test drive’ as described by the Worldwide Harmonised Light Vehicles Test Procedure, the quantum sensor traced the charge/discharge current from -50 A to 130 A and demonstrated a battery charge estimation accuracy within 1 percent. Of note is that the device can handle currents up to 1,000 amperes, which will come in handy when new generations of batteries and chargers are introduced, and a temperature range of -40 to 85 degrees Celsius.

These findings suggest that diamond quantum sensors could extend the driving range by about 10 percent. Alternatively, they would enable smaller battery packs while keeping the driving range the same. The latter option would reduce both the weight of electric vehicles and the energy to produce them. If all new electric vehicles would be equipped with the sensor right away, and current trends of electrification continue, the world’s transportation sector’s total emissions of carbon dioxide would decrease by 0.2 percent, the researchers estimate.