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Introduction: What is Thermal Runaway?

One of the most concerning and most publicized risks associated with lithium batteries is the potential for thermal runaway. Both lithium-ion and lithium-metal batteries unavoidably generate heat as part of their normal function, but if the battery is defective or if certain unusual conditions are present, the batteries can enter a self-perpetuating cycle of excessive heating known as thermal runaway that may lead to an explosion. Some of the most important aspects of battery testing involve understanding what factors may lead to thermal runaway, how effective built-in safeguards against thermal runaway are, and how the product will react if a thermal runaway event occurs. This category of testing is challenging, however, as thermal runaway can be just as dangerous to laboratory staff as it is to consumers.

Are Manufacturers Mitigating Thermal Runaway Risk Through Design?

The right chemical, electrical, and mechanical design considerations can make a dangerous thermal runaway event incredibly unlikely. High-quality batteries use technologies like advanced battery management systems (BMS) that protect against over-voltage during charge, overcurrent during charge or discharge, and under-voltage during discharge, all of which can lead to battery cell degradation. They may also incorporate thermal limits during charge or discharge. Sensible design choices can ensure that the battery pack doesn’t exceed the capabilities of the cell and can account for any potential variation in cell performance. Cell balancing helps cells within the pack age at the same rate and guards against any critical safety problems caused by low-performing cells.
The device that hosts the battery and the battery charger can also play vital roles in preventing dangerous thermal runaway events. Smart battery packs will use either proprietary or standardized communication protocols like SMBus or CAN to send data between device components. This enables the host device to monitor not only what the battery is doing, but how the battery is doing — and if it is necessary to step in and implement additional safety controls.

Failure rates for high-quality batteries that utilize such safeguards are so low that they’re difficult to precisely measure, but with demand for batteries increasing and the price of electronic goods decreasing, not all of the billions of batteries entering the market are high quality. We must contend with the reality that not all manufacturers are using the best safety technology available.

Is Third-Party Validation Enough to Ensure Safety?

Battery-powered technologies are usually subjected to system-level assessment during production or even in the field. Safety marks such as CE or UL indicate that battery-powered technologies have met certain minimum safety standards. Different markings and certifications involve different levels of scrutiny, but they do offer assurance to consumers that production facilities, supply chains, and the products themselves comply with minimum standard requirements.

Industry standards are extremely important to battery safety and have prevented countless dangerous product failures, but it is important to remember that no system is foolproof. Standards are updated very slowly and use broad language so they can be applied to a wide range of products. They are written and interpreted by human beings, which creates some unavoidable variation in how they are applied, and they may not address specific risks posed by newer technologies.

Even if a product conforms to industry standards, things can still go wrong. Products can suffer damage that causes them to function abnormally, or they can be used in ways that the manufacturer never intended. To fully understand and mitigate thermal runaway risk, manufacturers should subject their products to intentional thermal runaway testing, allowing them to understand what conditions cause it, what dangers it presents should the worst happen during use or shipping, and what additional design aspects are required for hazard mitigation.

Taking the Extra Step: Thermal Runaway Testing

There are two broad categories of thermal runaway testing: simulation and direct testing. Simulation uses computer models to predict the outcome of a wide range of scenarios, while direct testing applies different real-world conditions to a physical test subject to see what happens. Simulation can be very effective but tends to be both complex and expensive, so it is more often used by large organizations that keep their simulation programs and expertise in-house.

Direct testing is often more practical, but it presents its own challenges. First, it requires special tools and expertise to force a thermal runaway in a controlled setting. Batteries, after all, are generally designed not to explode. Once thermal runaway has been successfully triggered, the resulting heat, fire, explosion, fumes, and shrapnel can be dangerous to lab personnel and destructive to facilities and equipment.

To solve the second problem, ÌìÃÀ´«Ã½ designed and fabricated multiple temperature- and humidity-controlled, steel-walled blast chambers specifically for thermal runaway testing. Our experts designed the test chamber with multiple video cameras and data collection tools, a flowthrough multiport exhaust system to clear smoke and fumes, and a protected secondary control room for personnel and support equipment.

Safely forcing a thermal runaway event relies on the knowledge our technicians have gained over decades of work in battery testing. The techniques employed in each situation depend on the specific battery design, the customer requirements, and the scenario being replicated. For can cells that don’t have internal protections like a charge interrupt device (CID), fuse, or polyswitch (PTC), causing a thermal event by forcing the battery to overcharge is relatively straightforward. For pouch cells, overcharge protection can often be bypassed, but more complex batteries often have better built-in failsafe measures. Besides overcharging, another way to induce thermal runaway is overheating with an outside heat source. We typically use polyimide surface heaters to create a hotspot on the cell, which induces chemical breakdown and internal shorting. This method also has variables to consider, including the speed of heating and how the thermal mass of the battery will cause its temperature to differ from the temperature of the heat source.

The material surrounding the battery cell or pack can be included in testing, as customers often need to know what will happen if the battery enters thermal runaway during shipping or while in use in a finished product. What surrounds the battery can have a dramatic impact on the result, either by minimizing or exacerbating the consequences of a thermal runaway.
These scenarios represent only a few examples of the considerations involved in direct thermal runaway testing. It is a complex and delicate process with unique requirements and safety risks, but the data it provides is a valuable tool for protecting consumers and their property.

If you need to understand how your product will respond to a thermal runaway event, our experts are eager to answer your questions and explain our methods and capabilities. Contact us or browse some of our additional resources in the links below.