Differential Attenuation Characteristics And State Of Health Estimation Of LiFePO₄ Battery

LiFePO₄ batteries have the advantages of long life,large energy storage,etc.Therefore,it has been widely used in 3C digital,grid energy storage,electric vehicles and other fields.The battery management system?BMS?can effectively manage lithium-ion batteries and is an important component of lithium-ion batteries in practical applications.Among them,the State of Health?SOH?is one of the important contents of the battery management system,which can accurately and effectively identify the remaining available capacity of the battery,so that the battery pack can be fully utilized.In this article,the first step is to use multiple stress accelerated aging methods to perform aging tests on LiFePO₄ batteries.The experiment mainly uses four stress factors:charge rate,discharge rate,charge cut-off voltage,discharge cut-off voltage to accelerate the aging of LiFePO₄ batteries,and the purpose is to obtain the battery SOH data in a relatively short period of time from the begin of life to the end.Based on the data obtained from the aging experiment,the difference in capacity attenuation of the LiFePO₄ battery during accelerated aging was analyzed.And found that there is no specific linear relationship between the service life and the number of cycles of lithium iron phosphate batteries.At the same time,the capacity attenuation characteristics of the battery of the same model and the same model under the same conditions are also quite different,and these differences will accelerate the aging of the battery.Finally,the two SOH estimation methods are optimized and improved:The first SOH estimation method is the differential characteristic curve method.The evaluation method of the LiFePO₄ battery SOH extracted is mainly based on the aging characteristic parameters of the differential curve to realize the estimation of the LiFePO ₄ battery SOH.It is necessary to first filter the experimental data to filter the charging voltage curve,fit the dQ/dV characteristic curve according to the result,and then extract the characteristic parameters on the curve.Finally,the SOH is estimated according to the characteristic parameters.The second SOH estimation method is the probability density functio n.The method firstly applies the statistical probability density theory to extract the aging characteristics associated with the battery SOH.Furthermore,according to part of the constant current charging characteristic of the battery to be tested,its SOH is located by a formula.In addition,a sliding window optimization strategy is also used in this method to achieve the best SOH estimation accuracy.The above two methods have been compared with the estimation accuracy,and the results in the table show that the two SOH estimation methods have certain equivalence.During the estimation process,neither of them will produce an accumulated error.At the same time,they also have the characteristics of better single-estimation accuracy and characteristic charging interval identification,and can identify the SOH in the charging process of a lithium-ion battery.