Vehicles fueled by an alternate energy source are no longer a pipe dream.
They’re already here, and they’re on the cusp of being the dominant mode of transportation.
Production of electric vehicles (EVs) and hybrid electric vehicles (HEVs) is set to outpace those made with an internal combustion engine within the next decade.
Because of consumer demands and new government regulations, by 2030 most major auto manufacturers are expected to ditch the internal combustion engine in favor of lithium-ion battery packs for cars, trucks, and buses.
Once considered a barrier for alternative energy vehicles to enter the market en masse, thermal management -- and detecting the early signs of thermal runaway -- of lithium-ion battery packs remains a key component for the long-term viability of EVs and HEVs.
Though rare, thermal runaway in a lithium-ion battery can result in damage to the vehicle battery pack and the vehicle itself, as well as serious injury to any occupants.
Often triggered by a short circuit, overcharging, or other cell stress, a thermal runaway event occurs when a lithium-ion battery becomes overheated. The excess heat triggers a chain reaction in the cell that generates gas. If not mitigated, can spread to the rest of the battery pack, causing other cells to overheat and decompose. As the runaway takes hold and battery cells break down, the runaway causes the release of flammable gases, such as:
Other hazardous gases released during a thermal runaway include:
Once a thermal runaway begins, it’s difficult to stop as its chain reaction can cascade through a battery pack, often causing smoke and flames. With quick intervention, it’s possible to limit a thermal runaway’s impact on a battery pack, as well as the rest of a vehicle. While many materials used in the pack are designed to reduce the risk of fire propagation, once a cell vents gas, a hazardous condition exists within the battery pack that must be identified and dealt with to prevent the risk of fire.
Thermal management in EVs and HEVs requires a three-pronged approach:
In any case, stopping a battery thermal event happens through two methods -- active and passive thermal management systems.
Active thermal management relies on cooling systems that keep a battery pack at an optimal temperature.
When the cells start to heat up during charge or discharge, an active thermal management system extracts heat from the cells using air or cooling plates with conventional automotive coolants or even refrigerants to bring temperatures back down. It’s not unlike how a radiator keeps temperatures in check inside an internal combustion engine.
Passive thermal management systems are more focused on the later stages of preventing thermal runaway. Rather than keeping an affected cell cool, a passive system -- such as a heat shield or insulation -- blocks excessive heat from passing from an individual cell to the rest of a battery pack and continuing the chain reaction.
The idea behind passive thermal management systems is similar to compartmentalization as a form of fire protection in buildings. By containing a fire to an area, it doesn’t spread to other parts of the structure.
Like any other vehicle, keeping an EV or HEV running at peak performance involves constant monitoring of its systems -- that’s why electric cars, trucks, and buses have sensors and electronic controls that constantly monitor the battery condition and automatically provide appropriate cooling or heating as needed.
In the early days of lithium-ion battery pack thermal management, sensors measured heat -- the immediate, obvious sign of a thermal battery event. Now, sensor technology is taking a much different -- and more scientific -- approach toward thermal management, by monitoring for the gas release of a venting cell, the precursor to thermal runaway.
Hydrocarbon sensors and air pressure sensors have been trialed to manage thermal runaway, but aren’t accurate or robust means for thermal runaway management. Other materials inside a battery housing unit can interfere with a chemical sensor’s performance, skewing their readings and ultimately causing the device to fail. Air pressure sensors are vulnerable to variations in the pack’s venting system as well as altitude and temperature changes in the vehicle.
Physics-based gas sensors using technologies such as thermal conductivity or infrared spectroscopy, on the other hand, rely on gas physics to detect thermal runaway and allow for continuous monitoring after a battery thermal event begins. Using light-based and thermal conductivity technologies, gas sensors measure for CO2 and hydrogen -- the first gases vented by a battery cell during a thermal event. Unlike hydrocarbon and pressure sensors, these sensors are not affected by their environment in ways that degrade their performance over time, and their readings only measure the gases released at the beginning stage of a thermal runaway. These measurements are critical in providing the earliest possible warning to the battery control system, thereby reducing the risk that the thermal event will spread to other batteries in the pack, and providing a best-case scenario for safety and response.
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As EVs and HEVs begin to outnumber vehicles powered by fossil fuels, properly designed thermal management systems will remain a top priority for auto manufacturers.
The future of electric vehicles depends upon manufacturers providing a safe, reliable, and affordable vehicle with easy to manage recharging and other desirable features. Making these vehicles safe requires systems that not only prevent battery fire propagation but also provide early detection of cell venting as part of active vehicle hazard mitigation.
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