Better battery management: New technique measures electrochemical impedance of multi-cell stacked lithium-ion batteries

Researchers at Panasonic Corp. and Ritsumeikan University say they have developed a new battery management technology that measures a battery’s electrochemical impedance, which is an effective method of evaluating the residual energy of lithium-ion batteries in use. The technique is expected to be applied in applications characterized by numerous battery cells stack in series.

Panasonic developed the technology in collaboration with Ritsumeikan University Professor Masahiro Fukui. Panasonic developed a new battery monitoring IC (BMIC) test chip, measurement algorithm, and software. Ritsumeikan University researchers evaluated the performance using actual batteries.

The newly developed battery management technology makes it possible to measure electrochemical impedance using the ac current excitation, also called the ac superposition method, which measures the frequency characteristics of ac impedance by applying ac current while sweeping the frequency in a manner resembling that used for network analysis.

This new technology has the following features a temperature calibration technology that responds to changes in the battery operating temperature.
This contracts with conventional electrochemical impedance spectroscopy that is widely used as a non-destructive method for evaluating lithium-ion batteries. The conventional method requires an application-specific measuring instrument and a large thermostatic chamber that keeps the temperature of the battery constant.

Conventional BMIC measures the individual battery voltage of six to 14 lithium-ion battery cells stacked in series. By using multiple BMICs, BMS acquires battery cell voltage data from up to 200 cells connected in series. In the case of EV batteries, BMS calculates the remaining driving range and usable time by estimating the state of charge (SOC) and the state of health (SOH).

The newly developed BMIC test chip has a built-in electrochemical impedance measurement function using the ac current excitation method. The electrochemical impedance measurement is realized by 15 fully parallel ADCs and an ac current excitation circuit with pulse modulation from 0.1 Hz to 5 KHz and a complex-voltage/complex-current conversion circuit built in the BMIC.

Battery state estimation by conventional electrochemical impedance spectroscopy takes place through the measurement of the Cole-Cole diagram, a complex representation of the ac impedance where a frequency sweep trace is plotted on the complex plane. it is also called a Nyquist diagram. Ritsumeikan University researchers measured cylindrical lithium-ion battery cells using BMIC and measurement software developed by Panasonic. They confirmed the Cole-Cole diagram can be measured in the frequency range from 1 Hz to 5 KHz with the same accuracy as the standard measuring instrument used in the industry.

The electrochemical impedance of lithium-ion batteries is sensitive to temperature changes. Consequently, battery measurements typically involves a thermostatic chamber that maintains a constant temperature. Changing temperatures have made it tough to get a stable measurement of electrochemical impedance. So Ritsumeikan University researchers and Panasonic developed a temperature correction technology that measures the temperature of the lithium-ion battery during the electrochemical impedance measurement, corrects the temperature change of the impedance to the standard temperature, and draws it on the Cole-Cole diagram. This makes it possible to accumulate Cole-Cole diagrams normalized to the standard temperature in the database even when the environmental temperature varies.

Panasonic sees applications for the new technique in devices and vehicles that use multi-cell stacked lithium-ion batteries that include electric bikes, LSVs (Low-Speed Vehicles), construction and logistics machinery, etc. In the future, there are likely to be applications in electric vehicles and large-capacity storage batteries.

The Ritsumeikan University researchers and their joint research partner Fukui Laboratory made two presentations on the technique at the 60th Battery Symposium. This symposium takes place from November 13 (Wed) to 15 (Fri), 2019 by the Committee of Battery Technology, the Electrochemical Society of Japan, at Kyoto International Conference Center (Takaragaike, Sakyo-ku, Kyoto City).