What is thermal runaway in car batteries?

As defined by Wikipedia, thermal runaway “describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature . . . often leading to a destructive result.”

In the case of a car battery, thermal runaway occurs due to a battery cell failure of some kind, which then generates an increase in temperature, creating a chain reaction in which more energy is released, and even higher temperatures occur. This often leads to a destructive and very dangerous result. Here, we further explore thermal runaway, the effect, and how we can avoid the risks it poses. We look at the process itself and how that affects lithium-ion batteries. And we see if the issue can be detected, and indeed fixed.

What is thermal runaway and how can we avoid it?

Thermal runaway is a significant risk present in car batteries. The process starts owing to a battery cell failure of some kind, which then generates an increase in temperature, which creates a chain reaction in which more energy is released, and higher temperatures occur.

Damage continues to be done at high speed, most often leading to a destructive and dangerous result, such as the release of toxic and flammable gases, batteries exploding, or the culmination of a catastrophic high heat release fire event. To try and avoid thermal runaway, we need to understand how it can be detected, and fixed.

What happens during thermal runaway?

Thermal runaway occurs when an increase in temperature becomes an agent for a further increase in temperature. In the case of a battery, thermal runaway is due to battery cell failure of some kind – perhaps something as simple as the separator between the anode and the electrolyte breaking down.

This breakdown generates an increase in temperature, which then goes on to create a chain reaction in which more energy is released. The release of more energy causes the electrolyte to break down into flammable gasses.

Once the separator melts, the battery cathode further breaks down, fuelling the fire with oxygen and triggering an uncontrolled positive feedback loop, often leading to a destructive and potentially dangerous result – such as the release of toxic and flammable gases, batteries exploding, and even the culmination of a catastrophic high heat release fire event.

The risk of thermal runaway begins at 60°C and becomes extremely critical at 100°C. Temperatures then rise rapidly within milliseconds – creating higher, unforgivable temperatures of around 400°C – which leads to the last stage of the process: destruction and devastation.

What is the thermal runaway effect?

The thermal runaway effect is similar to, if not the same as, the ‘Runaway Reaction’, which is explained in Renato Benintendi’s ‘Explosions’ chapter of research paper, ‘Process Safety Calculations, 2018’ as, “a thermally unstable reaction system which shows an accelerating increase of temperature and reaction rate. A reaction that is out of control because the rate of heat generation by an exothermic chemical reaction exceeds the rate of cooling available.” The effect is basically caused by fast-building excessive heat within a battery, which sets off a chain reaction where temperatures continue to soar to anything up to around 400°C – when it’s too late for prevention.

What is thermal runaway in an Li-ion battery?

Thermal runaway is particularly prevalent in Li-ion, or lithium-ion, batteries.

Lithium-ion battery racks are structured to maximise energy storage density, which unfortunately also allows for fast fire spread once the thermal runaway effect takes over. Once ignited, fire can easily move to adjacent cells and construction materials, and become uncontrollable.

Lithium-ion battery fires are indeed notoriously challenging to fight. Gaseous suppression and water systems are just not effective.

While fire suppression systems can slow fire growth and heat release, they are not sufficient to provide complete extinguishment once thermal runaway has started. The most effective method of extinguishing these types of fires requires large amounts of water applied for many hours or even days.

The number of energy storage systems with lithium-ion batteries is projected to significantly increase over the next five years. Because lithium-ion cells can fail and explode — and often with little warning — it is more critical than ever to detect and prevent thermal runaway before the worst can happen.

Combining early off-gas detection with fire detection and suppression or inerting systems provides a holistic solution that delivers the early warning needed to help keep the energy storage systems industry operating safely and sustainably.

How do you detect thermal runaway?

To detect thermal runaway, we must first understand each stage of battery failure.

UK-based news portal, Energy Storage News, divided what they believe to be the four stages of lithium-ion battery failure into Prevention and Containment regions, to help explain how we may be able to detect and contain thermal runaway before catastrophe strikes.

Within the Prevention region sits Stage 1: Battery Abuse, and Stage 2: Off-Gas Generation.
During the Battery Abuse.

In Stage 1, thermal, electrical, or mechanical abuse results in cell damage, causing battery cell temperatures and pressures to increase. These increases should be noticeable to anyone monitoring the battery, and so the first potential stage of thermal runaway becomes detectable.

In the Off-Gas Generation stage, as cell temperatures and pressures rise, flammable gases vent from the cells. This is the critical point at which action must be taken to avoid thermal runaway and a fire event.

If off-gases can be detected (off-gases being the by-products of the chemical process that occur minutes before thermal runaway), and batteries are shut down before thermal runaway can begin, it is possible that fire danger can be averted.

An integrated solution makes this early intervention possible. One effective lithium-ion risk-prevention solution features monitoring and reference sensors that continuously check battery racks for the presence of lithium-ion off-gases. Reference sensors provide surrounding ambient air data to a controller, while monitoring sensors within the battery racks capture data relating to the air close to lithium-ion batteries. These sensors can detect lithium-ion off-gases in concentrations as small as one part per million (ppm) and are compatible with all current lithium-ion chemistries.

This risk prevention system is designed to disconnect batteries and prevent thermal runaway in less than five seconds. However, even after batteries have been shut down, flammable off-gases may still be present. Unless the area is sufficiently large or can be ventilated, these off-gases can still present a fire hazard.

This is where fire detection and suppression come in. If used at inerting concentrations, the fire suppression system can be used to inert the space after off-gases have been released. This can help prevent off-gases from reaching combustion levels in conjunction with oxygen. The point at which an inerting system is released requires careful consideration to be effective, and may require integration with other systems.

At regulation design concentrations, the suppression system can be used to help protect batteries from fire sources, such as Class A materials, and other electronic component failures before they become sources of heat that could ignite batteries.

Integrating off-gas detection with fire detection and suppression provides the early intervention required to help keep thermal runaway and fire danger at bay. The system does not require electrical or mechanical contact with battery cells and is essentially an upgrade for existing systems, allowing it to perform in live, working environments. Of course, if the area is small or can’t be ventilated, this is the moment when focus must very quickly shift from this detection-related region to fixing a thermal runaway.

How do you fix a thermal runaway?

If all stages of detecting thermal runaway have failed, fixing it becomes the focus – but really, to very little avail.

Continuing with Energy Storage News’ four stages of battery failure, Stage 3: Thermal Runaway marks the very end of the Prevention region and the start of the Containment region. During this stage, temperatures rapidly rise several hundred degrees and smoke is produced, this time also from the Lithium-ion battery failure. It all happens so quickly, and it is at this point that catastrophic failure is imminent.

Stage 4: Fire Generation is the first and only stage of the Containment Region. Following thermal runaway, fire ignites, and can easily move to adjacent cells and construction materials, becoming uncontrollable. This is the stage of catastrophic failure.