Managing Heat with Immersion
Southwest Research Institute
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By Dr. Andre Swarts
Batteries, like people, don’t want to be too hot or too cold.
Like the heroine of the classic bedtime story Goldilocks and the Three Bears, batteries prefer their temperatures to be “just right.” The lithium-ion batteries used by nearly every consumer device and electric vehicle on the road tend to degrade or age more quickly when they are outside of this “Goldilocks” temperature range. When it’s too cold or too hot, battery performance suffers.
For instance, during a hot Texas summer, drivers might hear the dreaded “click-click-click” indicating a dead battery. For gasoline-powered cars, this is typically a minor inconvenience, easily resolved with a set of jumper cables or a relatively inexpensive replacement. But with electric vehicles, it can be much more expensive and potentially dangerous to have a damaged battery powering a vehicle. As electric vehicles become more common, it’s important for longevity and safety reasons to find more effective ways to protect batteries from temperature extremes.
Fortunately, Southwest Research Institute is up to the challenge.
EV Thermal Management
In EVs, multiple battery cells are assembled into modules and packs in the smallest possible space, which limits air cooling and amplifies thermal management challenges. Battery temperatures are also affected by environmental conditions, such as ambient and road temperatures and the effects of rain, wind or snow.
A cold battery will use its own energy to generate heat, limiting the power available to the vehicle until it reaches optimal temperatures, and ultimately reducing range. Conversely, in hot weather, the vehicle’s cooling system reduces battery temperatures, consuming more energy and, again, limiting range.
Hot temperatures can also lead to battery failures and thermal runaway propagation (TRP). This is a dangerous, cascading failure in a battery pack where thermal runaway in one cell triggers a chain reaction, causing adjacent cells to overheat and so on. This can cause battery pack failures, fires and even explosions.
Other types of battery failure are often associated with manufacturing defects that lead to internal short-circuits or as the result of abuse, such as car crashes or penetration by a foreign body.
The earliest electric vehicles tried to manage temperatures by using air flowing directly from the cabin to heat and cool the battery. However, they soon determined that this direct connection between the battery and the people in the vehicle was not a good idea, and that technique has been relegated to history.
From left, Senior Research Engineer Daniel Robles, Staff Engineer Andre Swarts and Senior Research Engineer Swapnil Salvi developed an immersion cooling demonstration unit to provide customers with a simulated experience of battery abuse tests.
DETAIL
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Ethylene glycol coolant has been used for decades as an antifreeze agent in vehicle and heating, ventilation and air conditioning systems. As a liquid, it is colorless, odorless and combustible. Pure ethylene glycol freezes at −12.8 C (9 F), but, when mixed with water, its freezing temperature drops further, up to a point.
Liquid Cooling
These days, most electric vehicles use indirect liquid cooling to regulate battery temperatures. During this process, a thermal management fluid, or coolant, flows through channels embedded in cooling plates or cooling ribbons that are in contact with the battery cells. The coolant absorbs heat from the battery, transferring it away from the cells. The heated coolant is circulated to a heat exchanger to release the heat and then returned to the cooling ribbon to repeat the process. When the battery is cold, the same fluid is heated externally and uses a reverse process to heat the battery, using either waste heat or an auxiliary electric heater. The fluids used for these applications are typically a mixture of water and ethylene glycol, but pose the risk of hydrogen production through electrolysis should there be accidental contact between the coolant and the high-voltage components.
DETAIL
SwRI manages the Electrified Vehicle and Energy Storage Evaluation-II (EVESE-II) consortium, which is the current evolution of its highly successful EVESE program. Launched in August 2024, EVESE-II expands focus to include module and pack research, with an emphasis on immersion cooling research, test standards, safety testing and applications beyond electric vehicles, such as charging and vehicle-to-everything connected vehicle systems.
As battery power increases, so do the current requirements, or “C-rates,” which are the ratio of the charge/discharge current relative to its maximum capacity. C-rates are particularly significant for direct current fast charging (DCFC) technology. DCFC is the fastest way to charge an electric vehicle, allowing some EVs to reach an 80% charge in as little as 20 minutes. DC fast chargers convert high-power alternating current grid electricity to DC power and deliver it directly to the vehicle’s battery. Fast charging puts additional stress on thermal management technology and may overwhelm indirect liquid cooling, because performance is limited by the thermal conductivity of the cooling pathway. The heat generated from fast charging can degrade the lithium-ion batteries in EVs, so dissipating or managing that heat is vital.
About the Author
Dr. Andre Swarts is the program manager for the Electrified Vehicle and Energy Storage Evaluation-II (EVESE-II) research consortium and a leading member of SwRI’s Battery Systems Research and Innovation efforts. Swarts has more than 30 years of experience in the energy industry.