Battery charging system incorporating an equalisation circuit for electric vehicles

Abstract

Hybrid electric vehicles (HEVs) and electric vehicles (EVs) are gaining in popularity mainly due to the fact that unlike combustion-powered vehicles, they do not pollute with greenhouse gases and toxic particles. Most HEVs and EVs are powered by lithium-ion battery packs which have high power density and longer cycle lives compared to other battery types. Each pack is made out of many battery cells in series connected and due to manufacturing tolerances and chemical processes in individual cells each cell has its own electric characteristics. In order to achieve a balanced voltage across all cells, a battery management system (BMS) must be employed to actively monitor and balance the cells voltage. On-board battery chargers are installed in HEVs/EVs to charge the lithium-ion battery pack from the grid. This charger converts AC grid voltage into a controllable DC output voltage, but it adds weight to the vehicle, reducing the overall efficiency of an HEV/EV and also increasing its cost. The aim of researches in multi-functional power electronics is to design systems which perform several different functions at the same time. These systems promise cost and weight reductions since only one circuit is used to conduct different functions. An example is the electric drive in an HEV/EV. On one hand, it propels the car forward when driving, while on the other hand the battery can be charged via a modified electric motor and inverter topology. Thus, no additional on-board charger is required. This thesis describes a new multi-functional circuit for HEVs/EVs which combines the functions of voltage equalisation with grid charging. Compared to a drive system, the proposed circuit does not rely on an electric motor to charge the battery. Various battery chargers and equalisation circuits are first compared. Then, the design of the proposed circuit is described and simulation results are presented for charging and voltage balancing. An experimental test rig was built and practical results have been captured and compared with simulation results for validation. The advantages and disadvantages of the proposed circuit are discussed at the end of the thesis. Keywords- Multi-functional system, Battery charging, Voltage equalisation, Lithium-ion battery