Electric vehicle battery chargers : harmonics, modelling and charging strategies

Abstract

Electric vehicle is a viable solution for future transportation systems, as it is environmentally clean and energy efficient. However, in order for electric vehicles to become a reality, the issue of charging hardware and charging strategies must be considered prior to the introduction of the electric vehicles. This thesis concentrates on the investigations into the following issues: a) The harmonics generated by the electric chargers from the actual chargers of the electric cars currently available in Hong Kong. b) Investigations of the impact of the introductions of these chargers in a distribution system using a harmonic power flow program. c) To understand the performance of various battery chargers, several battery models were investigated and these were then simulated using PSPICE electronic circuit simulation package. A model was proposed that takes into account the non-linear characteristic of the self-discharge resistance, internal resistance during charging and discharging against the state of charge of the battery. The simulation results are then compared with experimental results to confirm the accuracy of the model. d) Investigations into the charging strategy that needs to be adopted to increase battery life. A new charging strategy was proposed and simulated using Saber simulation software and the results are compared with those obtained from the laboratory experiments for verification. e) A design of a new battery charger with the ability to ensure charge equalization in battery banks connected as a series string is proposed and implemented in the laboratory. Results from both simulation and laboratory experiments show that the proposed equalization charger can equalize the voltage across all batteries in the series string. It is envisaged that such a scheme will prevent overcharging and hence increase the battery life.