Monday, January 14, 2019

Keeping Electric Cars From Catching Fire

You don't have to search very hard to find stories of electric cars catching fire.  Unlike gasoline, which is pretty stable in bulk liquid form (but as explosive as can be as vapor), batteries have a problem with thermal runaway during operation.  Preventing a full thermal runaway condition is the focus of a lot of engineering effort

I can imagine some you saying, "Wait... what's thermal runaway?"
Each cell in a lithium-ion battery contains a flammable liquid electrolyte. If the cell short-circuits, the electrolyte can combust; pressure in the cell then rapidly climbs until the cell bursts and vents the flammable electrolyte.

Temperatures of the ruptured cell can increase to above 1,832°F (1,000°C). The rapid and extreme rise in temperature (thermal runaway) can easily propagate to nearby cells in a domino effect that has been dubbed thermal runaway propagation.

Thermal runaway generates smoke, fire, and even explosions. Occupants need as much time as possible to escape the vehicle if it happens.
As the number of electric vehicles continues to grow, the number of fires is going up.  I did a piece about burning Teslas five years ago, in February of '14.  At that time, Tesla seemed to have a catastrophic fire rate of about 1 in 4000 cars.  For perspective, at the same time, GM had just recalled 370,000 of their GMC Sierra pickups for a software fix that had caused eight fires, or 1 in 46,250, less than 1/10 the electric Tesla's rate.
Although thermal runaway is clearly life-threatening, so far there are no global regulation in place. Whereas China has implemented the GB/T 31485 standard (Safety Requirements and Test Methods for Traction Battery of Electric Vehicle), the UN has only proposed legislation. This leaves automakers with the choice of whether to design battery packs for their cars that can deal with thermal runaway. It’s up to their own risk assessment programs to determine how likely thermal runaway incidents are.  [BOLD added: SiG]
And that's a bit of a sticky situation.  One of those sayings in engineering is that it's all about compromise; if there were an ideal, perfect solution, everyone would be doing that and the problem you're working on wouldn't be a design choice.  In this case, adding things to batteries to make them safer might well impact the Electric Vehicles' other Achille's heel - inadequate range on a charge -  worse, because room in the battery pack will have to be sacrificed to make the cars safer.  Studies say that inadequate range is keeping some potential buyers away from EVs. 
Companies such as Morgan Advanced Materials have been researching and developing a range of thermal management protection materials and methods over the years. They provide more time for occupants to exit a vehicle and dissipate heat to cut the chances of a thermal runaway spreading uncontrollably. ...

There are three levels of protection engineers can design into batteries to reduce the effects of thermal runaway in electric vehicles. Namely, these are cell-to-cell, module-to-module, and battery pack level.
Cell-to-cell protection puts some sort of material between individual cells. It is the highest level of protection, but also the most challenging due to space constraints.  These materials typically undergo a phase change: solid to liquid or liquid to gas.  The phase change to gas seems to have the advantage of forcing open safeties already present in the battery pack to vent the hot gases.  The idea, though, is that the material keeps the heat and flames from propagating, breaking the "runaway" conditions.

Module-to-module protection puts insulation between modules to stop thermal runaway from "running away" and spreading to adjacent modules.  Naturally, the design depends on the battery module being protected, but treated paper (e.g. fish paper) has a long history in electrical components and could be used.  Module-to-module protection offers significant weight savings compared to cell-to-cell protection.  Lighter batteries in turn increase the range and lets the battery be more easily accommodated in the vehicle’s design.

Pack-level protection is the simplest and most affordable type. It is aimed at giving occupants more time to exit the vehicle. It provides little protection for the battery pack itself, so the car is likely to burn and take everything you own that you left in it when you bailed out.  On the other hand, if the aim is to save lives, it's far better than nothing.  Standard insulating paper is a common form of pack level protection, such as Superwool Plus Paper.

A large paper liner between the pack and the passenger compartment is the essence of pack-level protection.

In each of these protection schemes, cell-to-cell, module-to-module and pack, car makers can choose active or passive protection.  Paper barriers are obviously passive, but those aren't the limits of passive protection.  It could include separation barriers, heat sinks, and other ways to remove heat from the batteries by normal, heat transfer processes: conduction, convection, and radiation.  Active protection would utilize cooling technologies which consume energy themselves.  This would includes air, liquid, and refrigerant cooling. It also involves an external device that helps dissipate heat.  Active methods are generally more expensive and complex than passive techniques.

The best protection against thermal runaway places protective material around each cell in the battery.  It is also the most challenging, because all of the protective material consumes space that could go to passengers and cargo.


  1. The authors of that article don't seem to know that thermal runaway is an ancient term ("dubbed" implied it was just discovered and labeled). It can happen in any exothermic chemical reaction.

    Granted that Machine Design may not have many experts in chemistry or engineering, but at least when I went to school I learned this in 9th-grade chemistry class.

    Binary batteries are what is needed for vehicles. Mix two inert substances to produce electric current. Safe as tofu. Of course, we haven't come up with a chemistry yet that has enough energy density to make it practical, but electric vehicles should come with ejection seats until we do.

    1. A Tesla with ejection seats is a wonderful mental image. Especially if they're automatic, and not "pull the handle" ejection seats. People are driving along and suddenly go flying high enough for a chute to pop open, "whaaa!!!!!!" You'd sure like to not be under an overpass when that happens.

      I don't hold out much hope for EVs as practical for anything more than special purpose applications. I've heard of some batteries with an energy density approaching 1000 Wh/kg (Watt-hours per kilogram), vs. gasoline at 12,000 Wh/kg.

      We're far from being able to take a cross-country trip in an EV, and most people still don't buy their cars for their routine commuting or grocery store trips, they buy it to do everything they want including pulling the RV or a boat, or for taking it on a vacation trip.

      Machine Design is guilty of poor editing. The piece was written by a couple of folks from one of the companies mentioned in the piece, Morgan Advanced Materials. Many of these trade magazine articles are PR pieces written by the company working on the technology.

  2. I agree - EV's won't get far until they solve fundamental problems. I was in a museum recently that had 2 pre-WWI electric delivery trucks; they had an 80 mile range a century ago, and in recent years billions have been spent and yielded only incremental improvements.

    I heard once that Toyota said (or used to say?) that Prius' are not to be parked in parking garages because of the hazards from the battery pack going up - not just the toxic fumes, but also that if the battery pack got fully engaged, the fire was hot enough to destroy concrete and damage either the floor above or the one below.
    Have you ever heard about this restriction?

    1. No, I haven't heard of that, but it seems plausible.

      I've read reports that when electric cars burn they have to fight the fires for a long time, like up to 12 hours. (or was it 24?) They've assigned firefighters to follow a wreck towed to the junkyard to keep them from starting again and causing more trouble.

    2. Part of that is that most fire departments are not set up to fight metal fires. It may be starting to change, but it used to be that firefighters and military didn't try to put out lithium fires - they just put out anything that caught fire from the lithium and let the lithium burn itself out.
      I dealt with a couple of relatively large lithium batteries when I was a military civilian and that is what we did when they caught fire. Of course, we were in the middle of a field and had plenty of time and space to deal with it.

    3. I drove a towtruck for the CHP in the early 00's, and a couple of flatbed trucks were added to the fleet to handle electric vehicles. The wheel-lift trucks were told not to tow them, due to the regenerative effect of those wheels turning while being pulled. The proper way would have been to mandate using the dolly, just like you would if towing an AWD, but some of those drivers would ignore that rule...

  3. Time to rephrase that old Boeing saying, as: "If it ain't lead-acid, I ain't going!"

  4. Johnathon H beat me to it, but my maternal grandmother had an electric car for a while and found it very comfortable and easy to drive. I don't know how far it would go, but I gather it wasn't very fast. When Grandad traded it in for something else, Grandma drove the new car just like she did the old one - and threw rooster-tails of gravel all down the driveway. She told me that she gave him heck for not telling her that the new car was a bit different from the old one.

    Not that this has much to do with anything, but Great-grandfather had a Stanley Steamer which he enjoyed. He was an engineer by trade, and drove that car all over the U.S.