Vehicle Battery Pack Design and Considerations for Repurposing

Abstract

The market for hybrid and electric vehicles is expanding with the rise of gas prices and desires to curb climate change. With the creation of these complex systems comes the development of advanced battery systems which store and provide energy in the vehicle life stage. These batteries however have a limited lifetime in the vehicle, after which they can be used to provide energy in repurposed stationary energy storage applications. The objective of this thesis is to examine how electric vehicle batteries can be repurposed. The design of a hybrid vehicle battery pack, which uses mechanical topology optimization techniques to assist the designer in developing a weight-efficient design, is detailed. The battery pack under consideration is composed of Lithium-ion cells and the design techniques proposed can assist with the design of a lightweight repurposed energy storage system for a residential application. A design process for a repurposed battery pack is also proposed, which takes into account design steps from initial business/market predictions to installation of the assembly at a residence. This design process details a capacity fade model to predict battery state of health after the vehicle life stage, as well as a risk analysis which focuses on a design failure modes and affects analysis, fault tree analysis, and a code analysis. Finally, the design of two iterations of a repurposed battery pack bench test is documented with lessons learned for the design of future test benches and the full size repurposed pack. Lithium-ion battery packs are still relatively new to the vehicle market, and the ability for significant numbers of them to enter the repurposed market is a few years away. However, there are commercially available stationary battery packs that use this technology. As a result, there are a number of risks still evident in the design of a repurposed system as the relevant codes and legislation have not been written. Additionally, the nature of the collection, testing, and supply chain for the repurposed packs after vehicle use is currently unknown. It is recommended that more research be completed in the areas of battery state of health models as well as the business models for repurposed applications. Full-scale degradation research of packs is required in real-world vehicle settings, in order to understand exactly how the batteries degrade over a vehicle’s lifetime. As well, re-manufacturing firms need to understand how they can feasibly take used packs of uncertain quality to build the newly proposed assemblies while minimizing risk to the consumer and their own liability.