Accurate Remaining Available Energy Estimation of LiFePO4 Battery in Dynamic Frequency Regulation for EVs with Thermal-Electric-Hysteresis Model

Abstract

Renewable energy power generation systems such as photovoltaic and wind power have characteristics of intermittency and volatility, which can cause disturbances to the grid frequency. The battery system of electric vehicles (EVs) is a mobile energy storage system that can participate in bidirectional interaction with the power grid and support the frequency stability of the grid. Lithium iron phosphate (LiFePO4) battery systems, with their advantages of high safety and long cycle life, are widely used in EVs and participate in frequency regulation (FR) services. Accurate assessment of the state of charge (SOC) and remaining available energy (RAE) status in LiFePO4 batteries is crucial in formulating control strategies for battery systems. However, establishing an accurate voltage model for LiFePO4 batteries is challenging due to the hysteresis of open circuit voltage and internal temperature changes, making it difficult to accurately assess their SOC and RAE. To accurately evaluate the SOC and RAE of LiFePO4 batteries in dynamic FR working conditions, a thermal-electric-hysteresis coupled voltage model is built. Based on this model, closed-loop optimal SOC estimation is achieved using the extended Kalman filter algorithm to correct the initial value of SOC calculated by ampere-hour integration. Further, RAE is accurately estimated using a method based on future voltage prediction. The research results demonstrate that the thermal-electric-hysteresis coupling model exhibits high accuracy in simulating terminal voltage under a 48 h dynamic FR working condition, with a root mean square error (RMSE) of only 18.7 mV. The proposed state estimation strategy can accurately assess the state of LiFePO4 batteries in dynamic FR working conditions, with an RMSE of 1.73% for SOC estimation and 2.13% for RAE estimation. This research has the potential to be applied in battery management systems to achieve an accurate assessment of battery state and provide support for the efficient and reliable operation of battery systems.

A Trading Mode Based on the Management of Residual Electric Energy in Electric Vehicles

“…The cycling life is higher and the capacity degradation is minor for LiFePO 4 batteries, which can ensure remarkable cycling stability. Compared to other lithium-ion batteries, LiFePO 4 batteries maintain robust performance under high-rate charging and discharging conditions, and the battery's service life is effectively extended [23].…”

A Bidirectional Grid-Friendly Charger Design for Electric Vehicle Operated under Pulse-Current Heating and Variable-Current Charging

“…There are five reasons to support this replacement. Firstly, when the battery SOC is between 20% and 80%, the battery voltage can be approximated as a voltage source due to the small fluctuation [22], and the stability of the bus voltage can be freed from the control of the DC-DC link and the transient power fluctuation. In this situation, the DC-DC converter and the DC-AC converter have the capability to control the EV instructions and grid instructions separately.…”