As LiB cell is not a pure ohmic device but a complex electrochemical device with capacitive and inductive behaviors, its characteristic and performance will vary depending on the input signals, frequencies, and usage conditions. And as LiB cell is a non-linear system, its internal resistance (IR) depends significantly on variables, like aging (state of Health: SOH), state of charge (SOC), temperature, applying current rate and period. The IR is an intrinsic property of a cell representing its characteristic, performance, and health status at the measuring point. Therefore, its precise measurement is essential for its usage for cell characterization, quality control (QC), and grading-selection criteria for designing cell, module, pack, host system, and thermal management, health monitoring-prediction criteria. However, IR measurement is inherently complex and requires sophisticated procedures (1,2). Direct wave DC pulse and AC electrochemical impedance spectroscopy (EIS) are the prevalent techniques used for cell IR characterization (1,2). Most cell QC processes use the AC-impedance tool at 1.0 kHz with voltage, and IEC-62660 adopt a DC pulse, Hybrid Pulse Power Characterization (HPPC) method, and new proposals, like a multisine signals method and a pulse-multisine, have been proposed (1,2). However, the AC-impedance tool (1.0 kHz) cannot represent the dynamic behaviors of the cell and system compared to the DC pulse (3). In addition, some DC pulse test requires pre-testing conditions and a long test period; furthermore, there is no harmonized test technologies among the battery standards and some best practices of the LiB cell testing and characterization. To adopt a test method and condition as a QC tool for cell characterization that should be simple and not be time-consuming on pre-measurement condition and testing procedure to perform (3).

This research aims to propose simple IR measurement tools in which test results are comparable with each other’s and do not require a long period of pre-test conditioning time and a time-consuming test period. We evaluated four test methods of the cell IR measurement at four temperature conditions (0 ~ 5 ℃, RT (20 ~ 25 ℃), 35 ℃, and 45 ℃). In addition, we selected five types of cell groups (two 18650 cells, two pouch-type cells, and one prismatic cell) to evaluate the IR. The four IR measurement test methods are classified with DC pulse tools (simplified DC pulse method and HPPC) and AC pulse signal tools (Full range EIS and AC-pulse Impedance at 1.0 kHz). Test results studied the relationship between cells’ IR response behaviors by each type of pulse and the electrochemical mechanism of the cells. After comparing those test results, the best IR characterization tool and procedure will be proposed as a standard LiB cell characterization technology.

We found the simplified DC pulse method is simple and not time-consuming on both pre-test condition and testing procedure to perform and provide reasonable dynamic behaviors of the cell system compared to other methods. Figure 1 shows the trends of cell IR changes according to the experimental variables, applied current rate and frequency from the simplified DC pulse method. The blue color dot zone shown the optimum applied current rate and frequency and the red color dot zone shown the optimum frequency (3).

Reference:

1) Thomas R. B. Grandjean, Jakobus Groenewald, Andrew McGordon, Widanalage D. Widanage and James Marco, Batteries 2018, 4, 49; doi:10.3390/batteries4040049.

2) Mohammad A. Hoque, Petteri Nurmi, Arjun Kumar, Samu Varjonen, Junehwa Song, Michael G. Pecht, Sasu Tarkoma Journal of Power Sources 513 (2021) 2305193)

3) Jaesik Chung, Kwang Jung, and Giovanni Flores, 2020 NASA Aerospace Battery Workshop November 17 ~ 19 2020