Explosive: How to Qualify Battery Safety And Prevent Failures

October 1, 2019 | With increasing frequency, we hear about lithium ion battery explosions on the news. While the rate of field failures is statistically low at only 1 to 10 ppm, the impact of battery failures has often been life-changing for the victims involved. Higher energy densities and the use of lithium ion batteries closer to the human body are to blame for the severity of impact, explains Vidyu Challa, PhD, Consulting Manager, ANSYS-DfR Solutions.

Challa says the increasing concern around lithium-ion batteries means it is imperative for companies to have a risk mitigation strategy. Changes in the supplier landscape—a growing pool of lower tier battery suppliers emerging to meet demands for low cost batteries and scarcity of top tier suppliers due to their focus on the automotive segment—have increased the risk for companies that make battery powered products. Furthermore, with the rapid market growth for IOT devices, many companies without core battery competency, now have to build lithium-ion battery powered products, and manage their lifecycle.

Challa helps companies from a broad range of industries (medical, consumer, automotive, wearable, aviation, etc.) with battery powered products mitigate risk and prevent thermal events throughout the product life cycle. Lithium ion batteries have a narrow operating window, she points out, and these bounds must be respected throughout the cradle-to-grave lifecycle of the battery including manufacturing, assembly, storage/warehousing, transportation and use. ANSYS provides expertise in battery design and optimization, safety, manufacturing, qualification and root cause analysis. They do this through a combination of simulation and lab/test facilities.

Editor’s Note: Vidyu Challa will be hosting a workshop, How to Qualify Your Batteries to Prevent Failures & Thermal Events, during Cambridge Innovation Institute’s Battery Safety Summit, October 22-25, in Alexandria, Virginia. Mary Anne Brown, a Cambridge Innovation Institute program producer, conducted an email Q&A with Challa and shares their conversation here.

Please tell me a bit about your background and how you got where you are.

Vidyu Challa: After I finished a Master’s in chemical engineering, my first job was using ultrasound to detect defects in electronic components. I went on to get a PhD from the University of Maryland, where my work focused on reliability of electronic components and this philosophy called “physics of failure”. What that means is that a component or system’s propensity for failure depends on two things: 1. physical configuration and materials used, and 2. operating environment or lifecycle that the component/system is used in. Based on these two pieces of information, you can hypothesize what failure mechanisms would be most relevant for that product and when those failure mechanisms will initiate.

After I came out of grad school, I went to work for a battery startup. My work there was focused on trying to correlate battery prototype failures with process issues, and “physics of failure” came in handy. I brought on-board material characterization, failure analysis, and reliability capability. I introduced designed experiments, reliability modeling, and data analytics and we used these tools to optimize batteries based on active material, binder, and processing techniques. You can think of many levers with complex interactions that you can use to control the end battery properties. I led the scale-up, and commercialization effort for electrode processing. I brought that expertise with me when I joined DfR solutions, which recently got acquired by ANSYS. I have been helping customers in different market verticals work on a range of battery problems including manufacturing, design, lifecycle management, failure analysis, and failure prevention.

Tell us more about ANSYS and how it relates to batteries.

ANSYS is the leader in engineering simulation and its software is used to design products, as well as to create simulations that test a product’s durability, temperature distribution, fluid movements, and electromagnetic properties. When it comes to batteries, we are focused on safety/reliability challenges of integrating high capacity batteries into products. A key ANSYS motto is that simulation cuts down on product development costs and time-to-market. The traditional way of building product prototypes, running a test, failing, and fixing is expensive and time consuming. It’s even worse to wait until you have field failures. We unfortunately see this far too often with some of our customers.

At ANSYS we have multiple tools, like Fluent Battery Modeler and Twin Builder, to design safe and reliable batteries. One example is thermal management in electric vehicles. There are both safety and performance aspects here. What happens if a single cell gets really hot from a latent manufacturing defect? Will it propagate to surrounding cells and cause catastrophic failure, or will the thermal management design prevent propagation? This was one of the biggest lessons that the industry learned from battery failures on the 787 Dreamliner in 2014—to make sure the battery design is resistant to propagation. Meaning that if one cell fails, regardless of the reason, there is no thermal runaway or catastrophic failure from cell-to-cell propagation. Heat is also bad for capacity or performance. For performance reasons you also want cell-to-cell variation to be less than a few degrees.

You do a lot of work on field failures and helping companies qualify and benchmark battery suppliers. What are the industry trends you see and how are companies coping?

Lithium-ion batteries have become the rechargeable chemistry of choice due to their high energy density. We all love cell phones that have longer battery life or electric cars that have longer range.

There has been consolidation in the battery supplier space. Traditionally battery production was based in Japan among a few high-quality suppliers. In the past decade or so, China has emerged as the leading battery producer (and EV market leader) with 60% of the world’s battery production now in China. Due to the massive demand, there has been a growing pool of smaller suppliers, some with insufficient quality control practices.

One trend we have seen with our customers is scarcity of top-tier suppliers, who are focused on the automotive and transportation space. Non-transportation companies have had to move to lower tier suppliers. In some ways, that increases their risk. In the consumer and medical space, we see electronic gadgets that have moved very close to the human body and this increases the impact of a failure.

The automotive/transportation companies manage this risk in two ways: first, a very tight control of their supply chain to minimize latent manufacturing defects; and second, focus on effective thermal management to prevent thermal runaway, so even if a single cell fails, pulling heat away quickly can prevent a catastrophic failure.

In the small battery space of consumer, medical, and industrial applications, we see the gamut: companies who control their battery designs and supply chain very tightly and others who, owing to lack of awareness, treat a battery as a COTS component.

Can you talk about a recent publication of yours?

We had a battery failure analysis book chapter: “Failure Analysis and Quality Assessment of Batteries” in Linden’s Handbook of Batteries, Fifth Edition published by McGraw Hill this year. We discuss different battery failure analysis techniques, and point to an issue that I keep stressing in the battery courses I teach: avoiding single points of failure. Sometime the battery is not the source of failure but is the victim of an external short on the protection circuit board.

Can you share a recent example or use case that you found interesting or exciting?

Volkswagen’s all-electric race car recently broke the lap time record for electric vehicles at Germany’s famed Nürburgring Nordschleife racetrack. This was a racecourse with a lot of tight corners and steep inclines, extreme acceleration and deceleration, causing thermal fluctuations that could stress the battery and create  temperature differentials, potentially leading to shut down. To overcome these obstacles, Volkswagen Motorsports engineers leveraged ANSYS Fluents to evaluate different new cooling designs of the battery that did not compromise the vehicle’s aerodynamic design. Engineers also used ANSYS simulation to create a digital twin that virtually replicated the car’s performance on the track, proving that its battery could thrive on the challenging course and speed over the finish line. With ANSYS simulation solutions, engineers minimized the expensive and time-consuming trial-and-error testing process using a physical prototype.

Looking forward, what is needed as the battery industry progresses?

What the battery industry needs right now—and in the future—is success in five key metrics: safety, performance, life, cost, and environment. You always must make trade-offs to improve one metric over another. Current lithium-ion chemistries put high energy electrodes together with flammable liquid electrolytes. That poses some inherent safety risks, which are mitigated by having the right designs and thermal management solutions. While there is a lot of work being done on new materials, chemistries, and safety solutions, all with the aim of bringing new safer chemistries, commercializing a battery chemistry can take decades. That said, if and when solid state batteries do get commercialized, we may win on safety, but mechanical-stress-related failure modes in an all-solid matrix may dominate. We will continue to play the trade-off game, which is where simulation brings valuable insights.