The research suggests that ultrasound, or sound waves with frequencies higher than human hearing, may one day help electric vehicles better estimate the remaining charge in their batteries. Hongbin Sun, the study's lead author and an ultrasound engineer at Oak Ridge National Laboratory in Tennessee, stated that this method could also help detect unstable batteries on the verge of disaster, rapidly test battery quality during manufacturing, and determine which used batteries are healthy enough to be resold to reduce waste.
Estimating the remaining capacity of commercial lithium-ion batteries is currently a challenging task. For example, electric vehicles typically encounter about 10% uncertainty when estimating battery capacity. This, in turn, reduces their driving range by about 10% to ensure they remain within the battery's safe limits.
To find a simple, inexpensive, and non-destructive method to estimate the capacity of lithium-ion batteries, scientists studied ultrasound. As lithium-ion batteries discharge and recharge, underlying electrochemical reactions cause mechanical changes in the performance and structure of their components; in principle, ultrasonic scanning can detect these changes.
Previous studies on ultrasonic scanning of lithium-ion batteries typically used only a single ultrasonic frequency. In this new study, researchers examined multiple ultrasonic frequencies to understand the advantages and disadvantages of each frequency in estimating battery capacity.
In the experiment, scientists used a commercially available, off-the-shelf ultrasonic sensor, approximately 1.25 cm wide, to monitor a 2.4 amp-hour lithium-ion battery during charging and discharging. They tested relatively low frequencies of ultrasound: 750 kHz, 1 MHz, and 1.5 MHz, as the battery materials largely absorb frequencies higher than these. (For reference, humans can hear frequencies between approximately 20 Hz and 20 kHz.)
Researchers observed that the wave velocities at all three frequencies were related to charge states. The way the waves weakened also helped reveal phase changes in the materials within the battery during charging and discharging.
“Our proposed method provides a fast, low-cost solution for battery performance evaluation and understanding material phase transitions during battery charging and discharging,” Sun said. “For industrial-scale applications, it may be used to predict the state of charge of a battery and assess whether the battery has lost its ability to maintain a charge.”
Sun points out that a potential application of this method would be embedding low-cost piezoelectric ultrasonic sensors in electric vehicle batteries for continuous monitoring. He states that the technology currently has an uncertainty of about 5% when estimating battery capacity, roughly twice that of existing technologies. By improving sensor calibration and taking into account the temperature effects during the measurement process, "we expect the error to be reduced to around 2%," Sun says. "We expect this reduction in uncertainty to translate into improvements in EV range."
Furthermore, using ultrasonic scanning, "defective batteries are easily detected," Sun stated. Ultrasonic scanning "may also provide early warning of battery fires, as ultrasound is highly sensitive to temperature and material changes inside the battery."
Future research will need to test the technology on a variety of lithium-ion batteries, as well as a range of charging rates and more charging cycles; Sun says the method "requires a lot of additional work and technical validation before it can be commercialized."
