An EV battery pack has to keep out rain, slush and road spray, yet it still needs to let air in and out. Why the pack breathes, whether the car wash is a worry and how it copes with a failing cell.
What goes on inside the long flat box under an EV’s floor, from frosty mornings to the rare overheated cell
On a January morning an electric car sits in a driveway with frost on the windshield, after an evening spent splashing through slush and grey puddles of meltwater on the way home. Underneath it, between the wheels, runs a long flat battery pack that took all of that spray and rarely gets a second thought.
It is easy to picture the pack as a sealed box whose only job is to keep the weather out. Part of that picture is right, since water and grit have no business near high-voltage electronics. Yet a pack that never breathes would soon run into trouble of its own, which is why engineers spend a good deal of effort deciding when it should let air through and how much.
A closed box fights the weather
Air expands as it warms and contracts as it cools. A pack that heats up on the highway and then sits through a frosty night feels the air inside push and pull on its seals, and the same happens on a drive into the hills, where the pressure outside is lower. Over time that strain wears on gaskets, and a tired seal lets in humid air that, as the pack cools, turns into droplets inside the casing.
The difficulty is that the pack’s two needs pull in opposite directions. It has to let air move in and out as pressure changes, yet it must keep out rain, road spray and dust, and it rides low enough under the car to meet all three. Designers usually settle the conflict with a dedicated vent, a small part in the casing that lets pressure even out while blocking liquids and grit.
Several companies that specialize in controlling the flow of air and fluids develop these vents for carmakers, Vernay among them. The manufacturer, founded in 1946 and based in Griffin, Georgia, has designed a battery venting system for EVs whose vent carries IP68/IP69K ratings, the two that cover dust, prolonged submersion and hot high-pressure washing.
Should owners worry about the car wash?
Some new owners hesitate before taking an electric car through an automatic wash, worried that the jets will force water into the battery. Packs are sealed against road spray, and owner’s manuals generally treat an automatic wash as routine care, sometimes with advice for a particular model, such as folding the mirrors or avoiding brush washes.
The same manuals usually ask drivers with a pressure wand to keep the nozzle at a distance and not aim it at the underbody.
When a cell overheats
The breathing described so far is slow and gentle. The rarer scenario that worries engineers is a damaged or faulty cell that starts heating itself faster than it can cool down, a chain reaction known as thermal runaway. A cell in that state gives off hot gas, and if that gas had nowhere to go, pressure inside the casing would climb quickly.
For this reason packs also need an emergency exit, a path that stays shut in everyday driving and opens fast enough to release the gas when a cell fails. On most days, of course, nothing of the sort happens, and the vent does no more than let a cold pack exhale in the driveway.
Frequently Asked Questions (FAQ)
Is it safe to drive an electric car through a flooded road or deep water?
The advice is the same as for any car, which is to avoid deep water and to check the owner’s manual for the depth the vehicle can handle. An electric car that has been submerged, especially in salt water, should be inspected before it is driven, because the U.S. road safety agency NHTSA has warned that salt can cause short circuits inside the pack, sometimes leading to a fire weeks later.
What is thermal runaway in an electric car battery, in simple terms?
It is a failure in which a lithium-ion cell produces heat faster than it can get rid of it, so its temperature keeps rising on its own. The cell can then give off gas and smoke and may catch fire, and the heat can spread to neighbouring cells. Damage, an internal short circuit or overcharging can set it off.
What should a driver do if an electric car’s battery starts to smoke?
Signs include smoke, a burning or chemical smell and popping or hissing sounds from under the car. The driver should stop, get everyone out, move well away from the vehicle, upwind and uphill of the smoke, and call 911. Fire safety experts advise against trying to put out a battery fire with an extinguisher.
About the Author
