Reliability Analysis of Lithium-Ion Battery-Packs Used in Electric Vehicles

The lithium-ion battery-pack has been considered a good candidate for using in electric vehicles as an energy storage device owing to its high power-density, good stability, high charge-discharge efficiency and high-cycle life. The battery-pack usually consists of many battery-cells. The state of charge of the battery-cell degenerates along with charge-discharge cycles, and it results in the reliability issue of the battery-pack. However, there was little study on the relationship between the reliability and cost of a lithium-ion battery-pack. It should be an interesting research topic because over-enhancing the reliability of the battery-pack makes the cost increase and it is necessary to make a tradeoff between them. In this study, the state of health of a single battery-cell is modeled as a multi-state system in consideration of its uncertainty, and its reliability can be evaluated. Accordingly, the lithium-ion battery-pack becomes a large system consisting of many battery-cells and its reliability can be evaluated as well. In addition, fuzzy set theory is applied to take uncertainty of the state discretization into consideration. Based on the above modeling and the probability theory, universal generating function of the battery-pack can be derived, and the relationship between battery-pack reliability and cost of the battery-pack for various kinds of assembly can be disclosed. From the result of numerical analysis, it is found that with the same cost for the same numbers of battery-cells, the assembly having more parallel cells is more reliable than that of having more series cells.