Ironically, with flooding came fire—and no ordinary fire, either.
Five hurricanes strike the United States coastline every three years, according to the U.S. National Weather Service. These have the potential to cause great damage and destruction, as Hurricane Ian reminded Floridians in late September. The Category 4 hurricane cost over 150 lives and caused over $50 billion in damage.
A small part of that damage came in an unsuspected way: the storm proved disastrous to several electric vehicles (EVs), whose lithium-ion batteries ignited after prolonged exposure to saltwater. In Florida’s Collier County, the North Collier Fire and Rescue Department responded to at least six EV fires since Hurricane Ian and knew of others across southwest Florida, according to its October 11 Facebook post.
“There’s a ton of EVs disabled from Ian,” tweeted Florida’s Fire Marshal Jimmy Patronis on October 6. “As those batteries corrode, fires start. That’s a new challenge that our firefighters haven’t faced before. At least on this kind of scale.”
What causes this issue, and what can EV makers do about it?
Not Just a Normal Fire
The physics of the hurricane problem are easy to understand. EV batteries have a lot of stored energy. Seawater contains highly conductive salt. Together, those facts set the stage for an explosive short circuit.
But it’s worse than that: the heat and gasses generated in Li-ion batteries can cause long-lasting fires that are very difficult to extinguish. The North Collier firefighters reported one EV fire that reignited several hours after they thought it had been put out. In another instance, the South Trail Fire and Rescue District in Florida’s Lee County had to ask its Facebook followers to refrain from calling 911 about a Tesla submerged in a water-filled ditch, where the firefighters had left it to soak and ensure it wouldn’t reignite.
“When batteries of electric vehicles heat to the point of fire, it takes hours of copious amounts of water to extinguish,” the firefighters explained in an October 2 Facebook post.
There’s a reason it takes so much water to put out a Li-ion battery fire. Lithium is highly flammable and reacts with most extinguishing agents including water, carbon dioxide, and carbon tetrachloride. It bursts into flame at 180°C and reacts with almost all gases as well as with asphalt, wood, sand, asbestos, and more. Large lithium fires should be treated with dry sand, dry chemicals, soda ash and lime, or by moving the lithium to a safe location and letting the fire burn itself out, according to the U.S. National Oceanic and Atmospheric Administration’s CAMEO chemical database.
What Causes Saltwater Battery Fires?
Pure water is an excellent insulator, but impurities make it a conductor. Saltwater is much more conductive than pure water. Salt dissolves in water to create charged ions—positive sodium and negative chloride. When the water evaporates, the residual salt remains conductive and becomes a path for electric current to flow. In an EV battery pack, this could mean a short circuit between bus bars or other high-current components.
In practice, such a short circuit will not have the ideal zero resistance. If the actual resistance is relatively high, current will be low and heat will be generated slowly. However, since batteries are sealed, heat dissipation is difficult. Even slow heat accumulates in the battery and eventually triggers thermal runaway. A short circuit with low resistance—such as conductive salt bridges—is even worse, since it leads to much higher current than is safe for the battery. And when an EV is off, its battery’s protection circuits are off as well.
Oxygen, another crucial ingredient for fires, is readily available in Li-ion batteries. It’s in the electrolyte, separator, and cathode, allowing batteries to ignite even when they’re not exposed to air. The hottest cell will rupture and spread burning particles, molten aluminum, and copper from the electrodes. Outside the battery pack, fire is fueled by hydrogen gas, carbon monoxide, and hydrocarbon chains generated by the thermal runaway. The fire is constantly renewed by the molten aluminum and copper particles. Even when the fire is extinguished, the salty short circuits may still exist and lead to re-ignition.
Solving the Saltwater Challenge for EVs
EV makers have an unfortunate tradeoff when it comes to powering their cars. Many features that make batteries desirable for EVs also make them a greater fire risk. For example, high energy density is good for vehicle range and size, but in the event of a short circuit it will maintain a high current flow and create more heat.
The EV industry is also shifting towards higher voltage levels. Today’s EVs use around 400 volts, but the industry is on track to double that within the next few years. Higher voltage allows for lower current, lower heat, longer driving range, faster charging, and lower weight. However, higher voltage brings a higher risk. In the event of a short circuit, the higher voltage will run a higher current and generate more heat in the cells.
Although the EV fires caused by Hurricane Ian have raised concerns, advocates remain confident in the technology. “We are absolutely concerned about any safety issues regarding electric vehicles, it’s just that they must be put in context,” said Marc Geller, a spokesperson for the Electric Vehicle Association, in an E&E News article from October 21.
Geller continued: “One Tesla fire gets more news than 10,000 gasoline car fires, whether it’s because people are super interested in Tesla or because people are still fighting the battle to suggest that EVs are not ready for prime time or not good.”
The fact is that EV technology is still young, and engineers haven’t sufficiently addressed thermal runaway, the Achilles’ heel of Li-ion batteries. Most of the current efforts to increase EV safety are based on battery management systems, controllers, sensors, and protection circuits that are only active when the EV is on, not parked in a Florida garage. Powering these circuits when the EV is off is not an easy answer, as it will discharge the battery.
Another approach is to protect batteries from dirt, water, and humidity with a good seal, but even this isn’t as straightforward as it seems. Sealing can cause problems for cooling and make thermal runaway more likely.
Could the solution be switching to alternative battery chemistries, such as lithium iron phosphate or sodium-ion batteries? Perhaps. Both of these chemistries have less fire risk than current Li-ion batteries, but they also have drawbacks including lower energy density and higher weight.
Ultimately, the saltwater flooding problem is still waiting to be solved. It’s one more pitstop that automotive engineers must take on the road to perfect the electric vehicle.
