Summary:
Diehl Aerospace GmbH develops and produces emergency power systems and battery packs for aircrafts. Depending on certification requirements, nickel-cadmium- (NiCd), lithium cobalt oxide (LCO) or lithium iron phosphate (LFP) cells are used. They are monitored by an appropriate battery management system (BMS). Besides the functional safety, the determination of the remaining capacity, i.e. State of Health (SOH) and the actual State of Charge (SOC) are critical capabilities of the BMS.
LFP batteries have a nearly constant voltage across their entire capacity. Therefore, the DC voltage alone is not a meaningful indicator of the SOH and SOC. Since no current flows most of the time in an emergency power supply, the often-used Coulomb counting is not a useful method for SOH and SOC determination as well.
Objectives:
The objective is an impedance-based embedded system to provide online the status of the remaining capacity, i.e. the State of Health (SOH). This shall use embedded EIS-sensors such as DNB1101B [1] mounted on each cell and connected by a digital bus system to the BMS. Additionally, the state-of-charge (SOC) based on the impedance is required during idle (coulomb counting is used during discharge). For the health and safety condition as well as for temperature correction purposes, the internal cell temperature is needed, too.
Methods
Intensive tests have shown that the difference between the high frequency ohmic impedance and the low-frequency impedance of the LFP impedance (i.e. the Helmholtz capacity) is almost linearly related to the SOH of the fully charged cell [2]. Parameters of ageing data provided by lab measurements are implemented as look-up tables [3]. The determined SOH-values are monitored and used for a prognosis of the remaining useful time (RUL) under the assumption of a continuation of the use profile. Deviations of this forecast are forwarded as event notes.
The mid-frequency impedance provides information about the SOC of the cell expressed as pseudo-capacity or as dissipation impedance. However, the impedance of a cell also changes with temperature. Therefore, it is important to implement a temperature correction based on the Steinhart-Hart calculation [4]. The method uses the high frequency impedance to determine the internal cell temperature. Besides the temperature correction it can be used for thermal monitoring of the cell to detect abnormalities during operation.
In long duration cycling test runs these methods were validated on a multi-cell demonstrator with new and aged LFP cells.
References
[1] https://www.datangnxp.com/en/details/products/45
[2] State-of-Charge Monitoring and Battery Diagnosis of Different Lithium Ion Chemistries Using Impedance Spectroscopy, Peter Kurzweil, Wolfgang Scheuerpflug, Batteries 2021, 7, 17. https://doi.org/10.3390/batteries7010017
[3] Remaining Useful Life Prediction of Lithium-ion Batteries Based on Wiener Process Under Time-Varying Temperature Condition, Xiaodong Xu, Shengjin Tang, Chuanqiang Yu, Jian Xie, Xuebing Han, Minggao Ouyang; https://doi.org/10.1016/j.ress.2021.107675
[4] www.wikipedia.org/wiki/Steinhart-Hart-Gleichung/