Abstract
Battery impedance based state estimation methods receive extensive attention due to its close relation to internal dynamic processes and the mechanism of a battery. In order to provide impedance for a battery management system (BMS), a practical on-board impedance measuring method based on distributed signal sampling is proposed and implemented. Battery cell perturbing current and its response voltage for impedance calculation are sampled separately to be compatible with BMS. A digital dual-channel orthogonal lock-in amplifier is used to calculate the impedance. With the signal synchronization, the battery impedance is obtained and compensated. And the relative impedance can also be obtained without knowing the current. For verification, an impedance measuring system made up of electronic units sampling and processing signals and a DC-AC converter generating AC perturbing current is designed. A type of 8 Ah LiFePO4 battery is chosen and the valuable frequency range for state estimations is determined with a series of experiments. The battery cells are connected in series and the impedance is measured with the prototype. It is shown that the measurement error of the impedance modulus at 0.1 Hz–500 Hz at 5 °C–35 °C is less than 4.5% and the impedance phase error is less than 3% at <10 Hz at room temperature. In addition, the relative impedance can also be tracked well with the designed system.
Highlights
The research and development of electric vehicles (EV), i.e., pure electric vehicles and plug-in hybrid electric vehicles, is globally supported by governments and enterprises due to the energy and environmental crises
In order to analyze the measuring error of the designed impedance measuring prototype, the impedance of the battery cells in the battery module at the frequency of 500, 400, 250, 200, 125, 100, 80, 50, 40, 20, 10, 5, 2, 1, 0.5 and 0.1 Hz is measured with the electrochemical workstation, which is treated as the real impedance of the battery cells
It should be noted that there are abnormal data points at 100 Hz, which are due to interference from the AC power supply to the electrochemical workstation
Summary
The research and development of electric vehicles (EV), i.e., pure electric vehicles and plug-in hybrid electric vehicles, is globally supported by governments and enterprises due to the energy and environmental crises. Many state estimation methods for the battery management are proposed. Electrochemical impedance spectroscopy (EIS) is used as a direct indicator to character battery states besides estimating states with Kalman filter [1], Particle filter [2,3] and other state observers [4,5,6]. The impedance spectra can be fitted with lumped impedance models in a certain frequency range to characterize state of charge (SOC) [12,13,14,15] and state of health (SOH) [16,17,18,19,20]
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