Modeling and Implementation of a Hardware Efficient Low-Voltage-To-Cell Battery Balancing Circuit for Electric Vehicle Range Extension

Abstract

One disadvantage of electric vehicles is their limited driving range when compared to internal combustion engine vehicles. Battery packs are also a significant cost to electric vehicle manufacturers, and lithium-ion battery cells must remain within controlled voltage limits. Thus, the requirements for the electric system are to be cost effective, perform battery management, and make it as efficient as possible to increase its range.

Battery packs are typically constructed from around 100 battery cells in a series connection. During use of an electric vehicle, the battery cells become mismatched due to small differences in capacity. This effect is further amplified as the electric vehicle ages. Diverging cells cause issues during driving, since weak cells can limit the useable capacity of the vehicle. In order to use the whole capacity of the battery pack, and thus the entire range of the electric vehicle, the cells should be balanced. Strong cells should distribute their excess capacity to weaker cells during driving.

The thesis presents the design, modeling and implementation of a novel hardware-efficient battery balancing circuit. First, the theory behind battery balancing is presented. Next, existing battery balancing circuits are compared. Finally, the proposed battery balancing circuit is discussed. The design of the proposed topology is examined in detail. Simulations show that the circuit transfers energy between non-adjacent cells throughout the entire pack. Experimental work is performed on two custom printed circuit boards, a 12 cell lithium-ion module, and a 12V lead acid battery. The results confirm the function of the prototype.

The effect of the battery balancing circuit on driving range is examined with vehicle modeling simulations. A 2018 Chevrolet Bolt model is produced and capacity differences are given to each cell. The proposed topology balances the cells while driving, extending driving range on UDDS and HWFET drive cycles.