In the last decade and especially in the past few years, consumers worldwide have witnessed the meteoric rise of electric vehicles (EVs). Despite the many years that have passed since Tesla’s unveiling of the Roadster — its first entirely-electric sports car — in 2008, the development of electric vehicles does not look like it’s slowing down anytime soon. If anything, the adoption of EVs seems to have only accelerated.
Even as Tesla’s sales falter for the first time in a dozen years, US automakers as a whole have ended 2024 strong in its final months due to the huge demand for EVs. As highlighted by the New York Times, General Motors realized most of these gains, more than doubling its EV sales in the final quarter which places it second in electric vehicle sales only to Tesla. In China, automaker BYD reported a record number of EV sales in 2024, significantly narrowing its gap between Tesla. President Xi Jinping boasted these gains in his annual televised New Year address, proclaiming that “[f]or the first time, China has produced more than 10 million new energy vehicles [any vehicle fully or mostly powered by electric energy] in a year.”
However, even amidst the consistent growth, a fundamental aspect of EVs remains a concern to consumers: the safety of their batteries. In a 2025 study by Deloitte, battery safety was noted to be one of the top concerns for consumers in China, India, and South Korea with 37%, 37%, and 49%, respectively, listing it as one of their biggest concerns with EVs. In China, the prevalence of these safety concerns were on par with those related to driving range (38%), charging time (38%), and cold weather performance (37%). In the US, safety concerns (29%) easily exceeded anxiety about uncertain resale values (19%) and lack of knowledge (20%).
How justified these fears are, however, is still in contention for many. Although it seems almost routine now to see a dramatic EV fire headline, the rate of these fires are actually lower than the overall rate for all vehicle fires. In fact, according to the National Fire Protection Association (NFPA) on Vox, a staggering amount of car fires are still generated by conventional internal combustion vehicles, i.e. non-EVs. Nonetheless, when an EV does end up catching on fire, unique fire safety challenges are often presented.
For one, toxic and flammable gases are often released, introducing the risk of explosion. Furthermore, due to the large amount of energy that may be left in a damaged battery, the battery pack can reignite days to weeks even after the initial fire is extinguished.
An EV’s battery pack is comprised of smaller module(s) which are each comprised of even smaller individual cells. This modularized format allows EV manufacturers to easily design a battery with their desired electrochemical properties (e.g. capacity, voltage, power, etc.) simply by the combination of different cells and modules in series (higher voltage) or in parallel (higher current). For example, Tesla’s roadster features a battery pack of 9 “bricks” (modules) in series, with each “brick” containing 69 cells connected in parallel for a total of 6831 cells.
But this very property of segmentation has made EV batteries prone to a phenomenon known as thermal runaway. According to Lithium Fire Guard, thermal runaway can be understood in X events. First, a single cell is abused leading to heat being released. Second, the heat released begins to affect other cells, causing corresponding releases of heat in nearby cells. Third, if the process of heat generation becomes self-perpetuating, thermal runaway kicks in.
At this stage, the separators which prevent an internal short-circuit fail, even more heat is released as a result, and dangerous gases are vented. Fourth, sometimes, oxygen will mix with the released gases and lead to ignition, further propagating the release of heat in surrounding cells. This stage is often punctuated by sudden bursts of “jet-like, directional flames.” Fifth and lastly, in the most unlucky of cases, the released gases can suddenly explode. In their 2024 report on passenger EV battery fires, of the Lithium Fire Guard’s 511 verified battery fires, 489 exhibited cell ignition while just 22 ended with an explosion.
In their 2017 review paper about thermal runaway in electric vehicles, Feng et al. elucidates the chemical mechanisms different types of battery abuse induce that lead to thermal runaway. After a battery is subjected to mechanical abuse, its separator may be torn or penetrated and allows the anode and cathode to touch, resulting in a short-circuit and subsequent thermal runaway.
Similarly, thermal abuse can also lead to thermal runaway. Under extreme temperatures, the separator may simply collapse and also lead to a short-circuit.
Electrical abuse in particular is a dangerous factor. In an external short-circuit where the positive and negative sides are electrically connected due to water immersion or contamination, the sudden discharge of energy will generate internal heat. When overcharged, lithium dendrites — tree-like crystal structures — are formed on the anode (negative side) which can penetrate the separator and cause a short-circuit. Additionally, the lack of lithium on the cathode (positive side) may lead to the cathode structure completely collapsing, releasing heat. When over-discharged, similar problems emerge. The poles of the cell reverse and voltage becomes negative, resulting in an abnormal amount of heat generated.
Importantly, Feng et al. points out that after the separator collapses and thermal runaway kicks in, the process is basically unstoppable. With that in mind, it seems that there is nothing the consumer can really do to completely prevent thermal runaway. Overcharge/over-discharge are largely out of the user’s control, being primarily attributed to faults in the EV’s battery management system (BMS), and any risk of car crashes inevitably implies some risk of the resulting thermal runaway.
But there are some actions that may reduce the risk of thermal runaway. According to Road Safety, batteries should be kept out of extreme heat, maintained between 20% and 90%, and charged only when relatively cool. In the rare case a fire does occur, everyone should exit and move away from the vehicle and notify authorities.
Written by Andy Qi
