State estimation is a crucial part of battery efficiency, but it’s trickier than it sounds.
Batteries are ubiquitous in applications ranging from small electronic devices to high-power systems such as electric vehicles (EVs) and energy storage systems (ESSs). The application determines the safety and technical requirements of the batteries. It is not cost-effective to create advanced battery management systems for small electronic devices, yet when it comes to EVs and ESSs, such systems are crucial to ensure safe and reliable power.
With the capability to monitor, control and optimize battery performance, the battery management system (BMS) is key for high-performance batteries. Among the prime responsibilities of the BMS is to estimate the battery’s state of charge—but this crucial parameter is not always easy to measure. Two new research approaches may soon make the process easier.
Why Battery Management Systems Are Crucial
Batteries that operate in variable thermal conditions with frequent charging, deep discharging and high current peaks will have shortened life spans. A BMS can mitigate this. By measuring voltage, current, temperature and other variables, the BMS can optimize battery operation and control the charging/discharging process, thereby reducing the battery degradation process. The BMS keeps the battery system safe and reliable but also optimizes energy use (in an EV, this means more range).
An important role of the BMS is to estimate the real condition of the battery, including its state of health (SoH) and state of charge (SoC). SoH indicates the level of battery degradation and provides critical information about performance and lifetime. Estimating the battery SoH in real time is very important for automotive applications. It’s useful for maintenance and replacement schedules and can be used as a fault diagnosis to help prevent hazardous accidents. SoH information can also help manage EV energy consumption to extend the range and the lifetime of batteries.
SoC, which refers to the remaining quantity of energy available in a battery, is calculated as the ratio of available charge to full charge, from 0 to 100 percent. EV efficiency is highly dependent on accurate SoC estimates, and accurate estimates require accurate current sensors in the BMS. The challenge is providing accurate measurements for a wide range of current values—from milliamps to tens of amps to greater than 100 amps in fast charging scenarios. Unfortunately, sensors that measure hundreds of amps cannot accurately measure milliamps—but a new sensor from researchers at the Tokyo Institute of Technology is looking to overcome that restriction.
New Research Could Lead to Better Battery Management
In September, the Tokyo Institute of Technology (Tokyo Tech) published the results of a research collaboration with Yazaki Corporation to develop a sensor that can measure currents in a wide range with high accuracy.
Led by Tokyo Tech’s Mutsuko Hatano, the researchers built a prototype composed of two diamond quantum sensors that they claim can estimate battery charge within 1 percent accuracy in EV applications. With existing solutions providing 10 percent accuracy, the new sensor has the potential to boost battery efficiency by 10 percent, according to Hatano.
“We developed diamond sensors that are sensitive to milliampere currents and compact enough to be implemented in automobiles,” Hatano said in a Tokyo Tech news release. “Furthermore, we measured currents in a wide range as well as detected milliampere-level currents in a noisy environment.”
The two diamond quantum sensors measure currents in and out of the battery and use a differential detection technique to eliminate common noise. Together they can detect currents as small as 10 mA within an operating temperature range from -40°C to +85°C—more than sufficient for EVs.
Across the Pacific, researchers from the University at Buffalo have also been busy exploring a new approach to monitoring SoC: magnets. The researchers built a lithium-ion battery with a special magnetic material at one end. The magnetization of the material changes as lithium ions enter and leave it in the course of battery cycling.
“We can monitor the magnetism, and this enables us to indirectly monitor the lithium ions—the state of charge. We believe this is a new way to provide an accurate, fast, responsive sensing of state of charge,” said Shenqiang Ren, the lead researcher. The team published its findings in June under the title “Lithiating magneto-ionics in a rechargeable battery.”
While EVs get much of the attention when it comes to rechargeable batteries, these new approaches to SoC monitoring may enable better batteries in a wide range of applications. For engineers looking for that extra bit of battery power efficiency, this research is nothing short of energizing.
