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Crash tests shed light on improving Li-ion used in EVs

06 Jun 2013

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Lithium-ion batteries are ideal power sources for hybrid and electric vehicles because they are fully-rechargeable, lightweight, and is able to pack a lot of energy into a small volume. But these advantages also come with a big downside. Overheating and collisions may cause the batteries to short-circuit and even burst into flames.

To improve the safety of lithium-ion batteries, engineers have designed elaborate cooling systems to protect the battery packs.

Tomasz Wierzbicki, a professor of applied mechanics and director of Massachusetts Institute of Technology's (MIT) Impact and Crashworthiness Laboratory, believes that there may be ways to make batteries themselves more resilient. This could reduce the bulk of protective housing and in turn reducing fuel costs.

According to Wierzbicki, the first thing that needs to be done is for engineers to understand the mechanical properties and physical limits of existing batteries. Together with MIT postdoc Elham Sahraei, Wierzbicki studied the resilience of cylindrical lithium-ion batteries similar to those used to power the Tesla Roadster and other electric vehicles. The team subjected individual cells to forces mimicking frontal, rear and side collisions. Using data from these experiments, the researchers developed a computer model that accurately simulates how a battery can deform and short-circuit under various crash scenarios.

The researchers found that a battery's shell casing—an outer lining of aluminium or steel— may contribute differently to overall resilience, depending on the scenario. Making shell casings more ductile or flexible, the team said, may be one way to improve the safety of lithium-ion batteries.

Wierzbicki noted the team?s model may be used to design new batteries, as well as to test existing batteries. The model may also be incorporated into whole-vehicle simulations to predict a battery pack's risk of "athermal runaway," a term engineers use to describe cases of catastrophic fire and smoke.

"We are developing computational tools to redesign batteries so the new generation is more resilient," Wierzbicki said. "These batteries may be able to take much higher loads without getting into the thermal runaway that everyone's afraid of."

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