Flooding from Hurricanes Helene and Milton added billions of dollars in damage across the Southeast in September and October 2024, pushing buildings off their foundations and undermining roads and bridges. It also caused dozens electric vehicles and other battery-powered itemslike scooters and golf carts, to catch fire.

According to a compilation, 11 electric cars and 48 lithium-ion batteries caught fire after exposure to salty flood water from Helene. In some cases, these fires spread to homes.

When a lithium-ion battery bursts into flames, it is emits toxic fumes, burns violentlyand is extremely difficult to extinguish. Often, firefighters’ only option is to let it burn itself out.

Especially when these batteries are soaked in salt water, they can become “ticking bombs,” according to Florida State Fire Marshall Jimmy Patronis. This is because the fire does not always occur immediately when the battery is flooded. According to the National Highway Traffic Safety Administration, about 36 electric cars flooded by Hurricane Ian in Florida caught fire in 2022, including several that were towed after the storm on flatbed trailers.

Many consumers are unaware of this risk, and lithium-ion batteries are widely used in electric and hybrid cars, electric bicycles and scooters, electric lawn mowers and cordless power tools.

i am one mechanical engineer and I work to help solve battery safety issues for our increasingly electrified society. Here’s what all owners should know about water and the risk of battery fires:

The threat of salt water

The trigger for lithium-ion battery fires is a process called thermal runaway – a cascading sequence of heat-releasing reactions inside the battery cell.

Under normal operating conditions, the probability of a lithium-ion cell going into thermal runaway is less than 1 in 10 million. But it increases greatly if the cell is subjected to electrical, thermal or mechanical stress, such as short-circuiting, overheating or puncture.

Salt water is a particular problem for batteries because salt dissolves in water is leadingwhich means that electric current flows easily through it. Pure water is not very conductive, but the electrical conductivity of seawater can be more than a thousand times higher than fresh water.

All EV battery packs use gaskets to seal their interior space from the elements outside. Usually they have waterproof class IP66 or IP67. Although these ratings are high, they do not guarantee that a battery will be waterproof when submerged for an extended period of time – say over 30 minutes.

The battery packs also have various ports to equalize the pressure inside the battery and move electrical power in and out. These can be potential pathways for water to leak into the package’s casing. Inadequate sealing classes and manufacturing defects can also cause water to find its way into the battery pack if it is submerged.

How water leads to fire

All batteries have two poles: One is marked positive (+), and the other is marked negative (-). When the terminals are connected to a device that uses electricity to do work, such as a light bulb, chemical reactions occur inside the battery that cause electrons to flow from the negative to the positive terminal. This creates an electric current and releases the energy stored in the battery.

Electrons flow between the terminals of a battery because the chemical reactions inside the battery create different electrical potentials between the two terminals. This difference is also known as tension. When salt water comes into contact with metal battery terminals of different electrical potentials, the battery may short circuitinduces rapid corrosion and electric arcing and generates excessive current and heat. The more conductive the liquid that penetrates the battery pack, the higher the short-circuit current and corrosion rate.

Rapid corrosion reactions inside the battery pack produce hydrogen and oxygen, which corrode material from the metal terminals on the positive side of the battery and deposit them on the negative side. Even after the water drains away, these deposited materials can form solid short-circuit bridges that remain inside the battery pack, causing delayed thermal runaway. A fire can start days after the battery is flooded.

Even a fully discharged battery pack is not necessarily safe during flooding. A lithium ion cell, even at 0% state of charge, still has about three volts of potential difference between its positive and negative terminals, so some current can flow between them. For a battery string with many cells in series – a typical configuration in electric cars – the residual voltage can still be high enough to drive these reactions.

Many researchers, including my colleagues and I, are working to understand the exact sequence of events that can occur in a battery pack after it is exposed to salt water and lead to thermal runaway. We are also looking for ways to help reduce fire risks from flooded battery packs.

These may include finding better ways to seal the battery packs, using alternative, more corrosion-resistant materials for the battery terminals, and applying waterproof coatings to exposed terminals inside the battery pack.

What electric car owners should know

Electric cars are still very safe to drive and own under most circumstances. However, during extreme situations such as hurricanes and floods, it is very important to prevent EV batteries from being submerged in water, especially salt water. The same applies to other products containing lithium-ion batteries.

For electric cars, this means evacuating cars from the affected zone or parking them on high ground before flooding occurs. Smaller items, such as e-bikes and power tools, can be moved to the upper floors of buildings or stored on high shelves.

If you own an electric vehicle that has been submerged in water for hours to days, especially in salt water, public safety experts recommend treat it as a fire hazard and place it on open ground away from other valuable property. Do not attempt to charge or use it. Contact the manufacturer for an inspection to assess battery damage.

Often a flooded electric vehicle will need to be towed away for further inspection. However, since thermal runaway can occur long after immersion, the car should not be moved until it has been professionally assessed.

This article was originally published on The Conversation by Xinyu Huang on University of South Carolina. Read it original article here.