Applications for lithium-ion batteries are expanding in many industrial markets. Their use has made it necessary to reuse and recycle the electrical storage technology. Panasonic Corp. has developed a new battery management solution that measures electrochemical impedance, allowing it to assess the residual value of lithium-ion batteries in the devices.

Lithium-ion batteries required several years of theoretical and experimental study before being marketed. In recent years, the efficiency of the batteries, in terms of how much energy they can deliver compared to their size and weight, has been greatly improved. Researchers are facing additional challenges to continue improving power density, durability, costs, recharge times, safety, and recycling. Electrical performance monitoring through battery monitoring integrated circuits (BMIC) or battery monitoring systems (BMS) is the key to the ongoing success of applications with lithium-ion batteries.

Conventional BMIC measures the individual battery voltage from 6 to 14 lithium-ion cells stacked in series. By using several BMICs, it is possible to acquire data on the voltage of the battery cells connected in series, thus ensuring safe use. Furthermore, it calculates the remaining autonomy and the available time by estimating the state of charge (SOC) and the state of health (SOH). The BMS manages the entire array of lithium cells (single cells or entire battery packs), determining a safe operating area, within which the battery pack guarantees the best technical and energy performance.

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Figure 1: the new technology can be installed by replacing the BMIC

“Measurement accuracy is the most important thing for general BMS designers. The second is safety design such as functional safety. For automotive customers in particular, past usage in cars is also important,” according to Panasonic.

An essential measurement parameter for lithium-ion batteries is the electrochemical impedance. The electrochemical impedance of lithium-ion batteries is very sensitive to temperature changes and requires measurements made in a thermostatic chamber. Electrochemical impedance spectroscopy is a non-destructive method for evaluating lithium-ion batteries, a key to enabling their reuse.

Panasonic worked on this new technology in collaboration with Professor Masahiro Fukui of the University of Ritsumeikan. The company has developed a new IC battery test chip (BMIC), while Ritsumeikan University has evaluated the performance using real batteries with experimental techniques.

The new battery management technology enables the measurement of electrochemical impedance using the AC excitation method for lithium-ion stacked battery modules installed in operating devices. This is the answer to a sustainable society in which future lithium-ion batteries can be reused and recycled.

AC excitation works similarly in circuit layouts with precision amplifiers; it is advantageously used in transducer signal conditioning circuits to remove offset errors, eliminate 1/f of noise on the average, and eliminate the effects due to parasitic thermocouples. With a reduced sensitivity to 1/f noise, it is possible to produce a recognizable output signal with much lower excitation currents or voltage.

The electrochemical impedance measurement is obtained by 15 analog / digital converters and an AC excitation circuit with pulse modulation from 0.1 Hz to 5 kHz and a V/I conversion circuit integrated into the BMIC. Therefore, the BMIC can measure the electrochemical impedance of a running battery without significantly changing the configuration of the current BMS implemented in the design.

“This technology is widely used in battery development. We believe that residual value evaluation and end-of-life determination are important for battery reuse,” according to Panasonic.

“Ritsumeikan University,” the company continued, "has the technology and equipment to evaluate actual batteries and the knowledge to analyze the results obtained. We are a semiconductor development department and have little knowledge of battery evaluation. Therefore, we went with the cooperation of Ritsumeikan University. Also, we thought that the performance of LSI could be objectively evaluated by evaluating it with a third party organization other than Panasonic.”

The estimation of the state by electrochemical impedance spectroscopy is performed by measuring the Nyquist diagram: this is a complex representation of the AC impedance and the frequency on the complex plane. Ritsumeikan University measured the cylindrical lithium-ion battery cells using BMIC and measurement software developed by Panasonic, confirming the accuracy of the new technology in the range from 1 Hz to 5 kHz.

Lithium-ion batteries are a key technology for future forms of energy mobility and conservation. Their mass production is on the rise, mainly due to the trend towards electric vehicles. The main challenges required to offer high performance are those of greater autonomy, high recharging speed, and the reduction of maintenance costs, guaranteeing full eco-sustainability.

The technique employed by Panasonic is based on introducing a perturbation through an electric current source. This technology will find a strong use in applications that use lithium-ion battery modules with many battery cells stacked in series and in the next electric vehicles (figure 1).