Using alternate energy sources to fuel vehicles is no longer a pipe dream. These vehicles are already here and will be the dominant mode of transportation sooner rather than later.
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There will be higher production of hybrid electric vehicles (HEVs) and electric vehicles within the next decade than the production of vehicles with an internal combustion engine if trends continue.
Due to new government regulations and consumer demands, most major auto manufacturers are expected to switch from internal combustion engines in favor of lithium-ion battery packs for cars, trucks and buses by 2030.
Thermal management, and specifically, thermal runaway prevention, of lithium-ion battery packs was once considered a barrier to alternative energy vehicles entering the market en masse, but they remain a key component for the long-term viability of HEVs and EVs.
What Does a Thermal Runaway Look Like in an Electric Vehicle?
While it is uncommon, thermal runaway in a lithium-ion battery can cause damage to vehicle battery packs and to the vehicle itself, in addition to seriously injuring to any occupants.
A thermal runaway occurs when a lithium-ion battery becomes overheated and is often triggered by overcharging, a short circuit or other cell stress.
A chain reaction in the cell that generates gas is triggered by excess heat. This can spread to the rest of the battery pack if not mitigated, which can cause other cells to overheat and then decompose. The runaway causes the release of flammable gases as it takes hold and the battery cells break down. These include:
- Carbon monoxide
- Volatile hydrocarbons
During a thermal runaway, other hazardous gases can be released, including:
- Dimethyl carbonate
- Carbon dioxide
- Hydrogen Fluoride
It is difficult to stop thermal runaway once it begins because the chain reaction can cascade through a battery pack, often resulting in smoke and flames. It is, however, possible to limit a thermal runaway’s impact on a battery pack and the rest of a vehicle through quick intervention.
Many materials used in battery packs are designed to reduce the risk of fire propagation, however, once a cell vents gas, a different kind of hazardous condition exists within the battery pack that must be identified and dealt with to prevent the risk of fire.
Management for Thermal Runaway Prevention
A three-pronged approach is required for thermal runaway management in HEVs and Evs:
- Preventing runaway before it starts
- Identifying if and when runaway occurs within a cell
- Preventing runaway from spreading to other areas of the battery pack
There are two methods that can be used to stop a battery thermal event – active and passive thermal management systems.
Active Thermal Management
Active thermal management makes use of cooling systems to keep battery packs at an optimal temperature.
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 when the cells start to heat up during charge or discharge. It is similar to the way a radiator is used to maintain temperatures inside an internal combustion engine.
Passive Thermal Management
Passive thermal management systems are used to focus on the later stages of preventing thermal runaway. Passive systems, like insulation or a heat shield, block excessive heat from passing from an individual cell to the rest of a battery pack and continuing the chain reaction instead of keeping an affected cell cool.
The concept of passive thermal management systems is similar to the use of compartmentalization for fire protection in buildings. Containing a fire to an area prevents it from spreading to other parts of the structure.
Sensor Technology and Thermal Runaway Prevention
It is necessary to constantly monitor the systems of an EV or HEV to keep it running at peak performance, just like any other vehicle. This is why electric buses, trucks and cars have sensors and electronic controls to constantly monitor the battery's condition and automatically provide appropriate heating or cooling as and when required.
Sensors play a critical role in stopping a thermal runaway from spreading no matter what an EV’s thermal management system for its battery pack.
In the early days of lithium-ion battery pack thermal management, sensors measured heat, which is the obvious and immediate sign of a thermal battery event. There are now much different and more scientific sensor technology approaches to thermal management, such as monitoring a venting cell's gas release (the precursor to thermal runaway).
There have been trials for the use of pressure sensors and hydrocarbon sensors to detect thermal runaway, but so far they have not proven to be a robust or accurate way to detect thermal runaway.
A chemical sensor’s performance can be negatively impacted by the other materials inside a battery housing unit, possibly skewing their readings and ultimately causing the device to fail. Variations in the pack’s venting system can interfere with air pressure systems, as can temperature changes in the vehicle and altitude.
However, physics-based gas sensors that utilize technologies like infrared spectroscopy or thermal conductivity are reliant on gas physics to detect thermal runaway and allow for continuous monitoring after a battery thermal event begins.
Gas sensors measure for hydrogen and CO2 - the first gases vented by a battery cell during a thermal event - by using light-based and thermal conductivity technologies.
Gas sensors only measure the gases released at the beginning stage of a thermal runaway, and, unlike pressure and hydrocarbon sensors, gas sensors are not impacted by their environment in ways that degrade their performance over time.
These measurements reduce the risk that the thermal event will spread to other batteries in the pack and prove a best-case scenario for safety and response by providing the earliest possible warning to the battery control system.
Next-Gen Vehicles Need Next-Gen Thermal Management Systems
Properly designed thermal management systems will remain a top priority for auto manufacturers as HEVs and EVs begin to outnumber vehicles powered by fossil fuels.
It is necessary for the future of electric vehicles for manufacturers to provide affordable, reliable and safe vehicles with easy to manage recharging and other desirable features.
Systems that both prevent battery fire propagation and provide early detection of cell venting as part of active vehicle hazard mitigation are required to make these vehicles safe.
This information has been sourced, reviewed and adapted from materials provided by Amphenol Advanced Sensors.
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