State of Health Estimation of Lithium-Iron-Phosphate Battery Systems

Description

This study focuses on estimating the state of health (SoH) of a lithium iron
phosphate (LFP) battery system, which is crucial for assessing the value and
lifespan of new or used batteries in energy storage, grid support, and electric
vehicle applications. A proposed method for determining SoH based on comparing useful and nominal useful capacities in Ah and Wh, as well as total and
nominal capacity, has been presented. To validate the method, 200 charging
and discharging cycles over five months were performed. Three models were
developed to track battery behavior and one model to simulate degradation.
An extended Kalman filter has been used in the model to estimate the battery’s
non-linear parameters and filter the noisy measurements. The models revealed
that while estimating capacity using Coulomb and Watt counting proved difficult for the battery system that has been used, weighted least squares and
recursive weighted least squares methods showed promise for determining current capacity. Furthermore, an attempt to estimate the battery’s equivalent
series resistance was performed, but no conclusion could be drawn due to limited knowledge of battery parameters. The findings highlight the challenges
of modeling and estimating the SoH of used batteries and suggest the need
for more targeted experimentation to improve battery modeling and estimation
accuracy.

Period	14 Apr 2023
Examinee	Carl Fredrik Johan Spångberg