Floridians battered by Hurricane Idalia this week may not have expected another threat — that floodwaters could cause their cars to suddenly burst into flames. 

Yet that's exactly what happened when two electric vehicles caught fire after being submerged in saltwater churned up by the storm. Firefighters in Palm Harbor, Florida, cited the incidents, both of which involved Teslas, in warning owners that their rechargeable car batteries could combust if exposed to saltwater. 

"If you own a hybrid or electric vehicle that has come into contact with saltwater due to recent flooding within the last 24 hours, it is crucial to relocate the vehicle from your garage without delay," the department said in a Facebook post. "Saltwater exposure can trigger combustion in lithium-ion batteries. If possible, transfer your vehicle to higher ground." 

The warning also applies to electric golf carts, scooters and bicycles, with lithium-ion batteries potentially sparking a fire when they get wet. More specifically, salt residue remains after the water dries out and can create "bridges" between the battery's cells, potentially creating electrical connections that can spark a fire. 

Lithium-ion battery packs consist of a group of cells inside a compartment and contain a flammable liquid electrolyte. EV and plug-in hybrid vehicles have about 1,000 times more cells than an e-bike, according to a report by the CBS News Innovation Lab. Higher energy batteries with more cells are at greater risk of failing.

In Florida, fire crews were towing one of the vehicles, a Tesla that had been submerged in Pinellas County, Florida, when it abruptly went up in flames, Palm Harbor Fire Rescue training chief Jason Haynes told CBS MoneyWatch. He said combustion can occur well after a car is exposed to saltwater and emphasized the importance of moving potentially damaged vehicles out of garages and away from nearby structures. 

Tesla warns car owners about the risks of vehicle submersion and advises against driving a car that has been flooded. "Treat your vehicle as if it has been in an accident and contact your insurance company," the company says in its guidance for handling a submerged vehicle. 

"Safely tow or move the vehicle at least 50 ft (15 m) from structures or other combustible materials such as other cars and personal property," the company adds.

Fires can ignite weeks after flooding

Fires linked to a soggy lithium-ion battery don't necessarily occur immediately after exposure, underscoring the importance of having a vehicle that has taken on water inspected by a professional.

"And it can take from days to weeks later," Patrick Olsen, spokesperson for Carfax, which sells new and used vehicles, told CBS MoneyWatch.

Some EV owners are unaware of the risks from flooding, he added. "I have heard EV owners say, 'I don't have an engine that can be flooded so I can drive in deep water.' That's not the case."

Andrew Klock, head of electric vehicle training for firefighters for the National Fire Protection Association, explained that electric vehicles are not inherently more dangerous than gas-powered cars and trucks provided that everyone — from motorists to emergency responders — know how to deal with flooding incidents. 

Even firefighters may not know how to properly extinguish an EV battery fire. "Often they don't direct the water to the proper place," which for electric cars would be under the chassis, where batteries are located. 

"If you don't do that and keep dumping water on top, it won't necessarily make its way down to where the battery is," he said.