Advanced battery management system for electric vehicles with modular battery packs

Ashraf, Adnan (2024). Advanced battery management system for electric vehicles with modular battery packs. University of Birmingham. Ph.D.

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Abstract

Higher voltage systems are becoming more common in electric vehicle (EV) technologies due to the convenience of quick charging. More series-connected cells in the battery pack are required due to the voltage rise. The two main issues raised by this transition are maximising power loss in the traction inverter due to higher voltage and guaranteeing that the voltage/ state of charge (SOC) remain balanced during rapid charging. The purpose of this thesis is to investigate these issues and provide recommendations for improving the effectiveness and efficiency of high-voltage EV battery packs. There are several cell balancing methods in the literature, but they don't offer flexible DC link for a traction inverter. For the purpose to provide a flexible DC connection voltage and ensure cell balancing for quick charging, this thesis offers a modular battery pack that uses a bypassed cell balancing topology. In this balancing method, the battery pack’s each module is linked to a pair of switches, typically MOSFETS. These switches enable the series or bypass connection for each module in a battery pack. The SOC estimation of a battery cell is the most challenging part of the battery management system. Thus, the voltage-based control algorithm is implemented to eliminate the need for SOC estimation, and the SOC measurement results indicate that lithium-ion (li-ion) cell voltage provides a good approximation of the SOC. The default li-ion battery cell model in MATLAB Simulink is optimised by using actual parameters of the cell. The parameters tuning of an equivalent circuit model of an induction motor is performed to develop an approximately realistic motor model.
The battery charger is designed to adjust its supply voltage because the voltage of the EV battery pack varies by using the proposed bypass cell balancing topology. Field-oriented control (FOC) is implemented to control the speed of the motor and provide the voltage amplitude required according to the speed of the motor. The controller adjusts the DC-link voltage according to the motor speed and provide adaptable DC-link voltage for the traction inverter while maintaining the voltage balance of the battery pack module.
The performance of the proposed modular battery pack is tested for voltage balancing, controller effectiveness, residual imbalance, and providing variable DC link voltage for a dynamic load condition using hardware in the loop and simulation to test the different scenarios of initial SOC or voltage imbalance. The result analyses of charging and discharging battery pack shows the proposed bypass balancing method achieved approximate zero SOC/voltage residual imbalance between series connected modules for rapid charging of 2C rate. The power loss assessments at the battery pack and traction inverter levels shows the proposed method reduce the overall system power loss and improved power efficiency.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):