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Estimation of Probability Distribution of Li-ion Battery Performance for Electrically-propelled Vehicles

Article of Honda R&D Technical Review Vol.29 No.2

Summary

The usage and performance status of Li-ion batteries installed in hybrid electric vehicles were collected by means of fault diagnosis devices. Analysis of the collected data showed the existence of a margin between the Li-ion battery capacities in users’ usage conditions and the estimated capacity degradation line that is foreseen during development to show the most rapid degradation. The distribution of the actual Li-ion battery performances in the users’ usage conditions is composed of the initial performance distribution, the degradation distribution created by different usage patterns, the degradation distribution of Li-ion batteries within an identical set, and the performance calculation errors made by the electric control units. The capacity margin occurred as a result of the estimated performance degradation line foreseen during development being calculated under conditions in which the distributions of the above performance factors were simultaneously positioned at severe extreme values.
A formula, which reflects the actual distributions and the correlations between the distributions, was created to calculate the probability distribution of performance degradations of Li-ion batteries. Usage data and capacity data on 126,742 cars of the 2013 model FIT HYBRID for the Japanese market were used to verify the validity of the formula that was created. The results show that the probability distribution of real capacity at 1.5 years after a vehicle is registered agrees with the probability distribution of estimated capacity after 1.5 years as calculated using the created formula. This verified the validity of the formula. This also successfully quantified the position of the capacity estimated using the estimated capacity degradation line foreseen during development with respect to the estimated probability distribution of performance after a target use period as 9.6σ. As it became possible to calculate the probability distribution of performance after degradation, this enabled optimal cost design of Li-ion batteries, bringing out the maximum vehicle performance of hybrid electric vehicles during the target use period.

Reference

(1) Takemasha, K., Miyashita, T., Akamatsu, K., Hada, S., Joko, M.: Accurate Capacity Estimation and Forecast of Li-ion Battery for BEV Using Telematics, Honda R&D Technical Review, Vol. 28, No. 1, p. 153-159
(2) Miyashita, T., Kawamura, M.: Customer Usage and Durability of Li-Ion Batteries for Honda’s Electrically-Propelled Vehicles, the 17th Annual Advanced Automotive Battery Conference, (2017)
(3) Ichimura, M.: A Study of Life Characteristics of Small Lithium-Ion Batteries, NTT Builiding Technology Institute 2005, p. 1-7, (2005)

Author (organization or company)

Takuya MIYASHITA(Automobile R&D Center)、Masayuki KAWAMURA(R&D Center X)

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