Estimation Technology of Residual Life of HEV Batteries Using Electrochemical Impedance Spectroscopy
Authors: Ito Shinichiro , Koba Daisuke , Nishi Hiroki , Yao Takeshi , Mukoyama Daikichi , Nara Hiroki , Tsuda Shingo , Momma Toshiyuki , Osaka Tetsuya 
Published: February 2020

1.Introduction
Recently, hybrid electric vehicle (HEV) is getting popular in the car market.Nickel-metal hydride (Ni-MH) batteries are used most in the HEV market. Capacity of the Ni-MH batteries which are used in the market is decreasing as well as other rechargeable batteries like LiB and VRLA. Initial battery capacity is limited by positive electrode because we designed that negative electrode capacity is larger than positive electrode. So, we use middle range of SOC (state of charge) for negative electrodes.
The battery capacity is decreasing by a lot of factors depending on the battery usage condition like temperature and duration. Capacity decrease by the negative electrode is mainly caused by the SOC shift. If the battery is used in this condition (SOC shifted) continuously, negative electrode should become over-discharge. And then, this leads to the degradation of battery performance. That is why we need to develop not only measurement method of battery capacity, but also estimation method of the SOC for negative electrodes.
Then we have reported the technology by using electrochemical impedance spectroscopy (EIS) for detecting the negative electrode SOC. And we also reported practical use by using this method. In particular, the imaginary part of nyquist plots of the negative electrode at SOC0% is greatly changed, so we could make life diagnoses by estimating the negative electrode SOC at low battery SOC. It is possible in the inspection process in which the battery SOC can be controlled [1][2].
If we detect the positive and negative electrode SOC for middle range, we can examine whether batteries are reusable on board, or we can provide the information of residual life of batteries to users. In this study, we were pursuing the possibility of the negative electrode SOC estimation by measuring the EIS under various temperature environments in the middle range of SOC.

2. Experimental
In order to measure the potential of negative electrodes, we used Hg/HgO reference electrodes. SOC 0% of negative electrodes is defined as -0.7V vs. Hg/HgO.The EIS spectra were obtained with 0.15C of AC signal in the frequency range of 100kHz to 1mHz at 0-45oC. Various impedance parameters were calculated by equivalent circuit fitting.

3 Result and discussion
We confirmed the relation between impedance parameters and negative electrode SOC at various temperatures. Fig.1 shows fmax　which means the frequency at the top of semicircle of nyquist plot was found to have a correlation with negative electrode SOC.Also, we could see correlation between fmax and the SOC at various temperatures (0-45 ℃). We found that fmax were getting higher at low SOC and high temperature.In the range of low SOC and a high temperature, it is known that the reaction rate of the negative electrode is increased. So it was suggested that fmax is correlated with the reaction rate.
We found the possibility of the estimation of negative electrode SOC aiming at estimating residual life by using fmax parameter.
4 References
[1]H. Nishi, D. Koba, S. Ito, T. Yao, D. Mukoyama, H. Nara, T. Momma, T. Osaka, The 56th Battery Symposium in Japan, 1M25 (2015)
[2]D. Koba, H. Nishi S. Ito, T. Yao, D. Mukoyama, H. Nara, T. Momma, T. Osaka, The 56th Battery Symposium in Japan, 1M26 (2015)

Figure 1