After about 200 million miles driven (extrapolating from anecdotal data and the culmulative total of 200,000 model 3's sold so far), there has been ZERO battery fires so far: https://teslamotorsclub.com/tmc/threads ... es.141231/
It has a long way to go, before it can catch up to the Leaf's billion+ miles with only 1 fire, but it's on its way!
Good news. Does anyone know if Tesla added back some/all of the individual cell protection features to the 21700s that they had Panasonic remove from the 18650 cells on the Model S/X, to boost the specific energy/energy density and lower the cost? No other car manufacturer has been willing to do that for safety/liability reasons, but Tesla had to make a splash with the Model S so was willing to rely on outside the cell protection and accept the greater risk of fires. Here's a description of those changes from back in 6/2013 on TMC, by member CapitalistOppressor:
Regardless, the primary patent (the first one linked) basically removes most (all?) of the safety features in the commercial 18650. Here are diagrams of the interiors -
First (conventional commercial battery design) -
Cell cap assembly with recessed terminal and enlarged insulating gasket - diagram, schematic, and image 02
Second (Tesla's amazing, exploding, non-functional battery design) -
Cell cap assembly with recessed terminal and enlarged insulating gasket - diagram, schematic, and image 01
It should be obvious how much this simplifies the manufacturing process. Many complicated features and manufacturing steps are just deleted, and the safety systems are handled by the central battery control system. The battery is also considerably lighter because of the ability to use aluminum (the cost difference between that and steel should be negligible at this scale). Besides making it cheaper and lighter, these changes also facilitate Tesla's manufacturing process and pack integration. I'm impressed.
The second patent deletes the plastic film on the exterior of the battery, leaving bare metal. This cuts both weight, and improves the ability of Tesla to do thermal management on the battery (a plastic film is bad for preventing fires apparently). This change also makes the battery explode (technically, it will short circuit). Non-obvious improvement. . . .
There is a related patent that adds a gasket (basically a thin film at the cylinder edges) which is enough to keep the batteries from shorting out (which is one of the purposes of the plastic cover) in the very controlled environment that Tesla has inside of its pack. I didn't link it because its inclusion is almost immaterial in terms of cost, weight or functionality, though it does make the deletion of the cover possible. It also doesn't prevent the battery from exploding. Tesla's pack and manufacturing process does that. Non-obvious.
Tesla's Amazing Cell Level Thermal Management - Battery Fire All But Impossible
Here is the critical thermal management patent that is almost certainly being implemented, that also includes key information on how the cells are integrated into the pack, as well as the changes to the interior and exterior of the cell to prevent thermal runaway -
Cell Thermal Runaway Propagation Resistance Using Dual Intumescent Material Layers - Patent application
The key feature is that after pack assembly, the cells and entire interior of the pack are sprayed with 2 layers of intumescent material. For those not familiar with it, its basically a material that when exposed to a heat source will absorb that heat, and then undergo a chemical reaction causing it to expand.
An earlier patent by this research team used a single layer, and didn't describe how it was to be integrated into the manufacturing process. In that patent, the single layer would suck heat out of the battery (significantly delaying or stopping thermal runaway) and then expand, keeping the battery thermally separated from other components. Once it got hot enough, it would char and harden. Once this char formed it created a hard thermally resistant cylinder which would direct any heat which managed to burst through the battery shell vertically through the cylinder, and away from surrounding batteries.
In the "finished" patent this charring layer is the second layer, while the first layer is able to absorb heat quicker (thus increasing the chance of the battery not bursting) and then transfer its heat to the second layer, which would provide the final barrier effect described above.
In addition, they describe a manufacturing process which would coat the interior surfaces of the cell with their own intumescent barriers, which would hopefully halt thermal runaway before it ever gets to the exterior of the battery. Again, because this just adds a quick (and cheap) step to the manufacturing process (where the battery is dipped or sprayed) it seems highly likely to be in use.
The pack assembly process is detailed, and basically the individual cells are integrated into the pack, and then the two layers of intumescent material are sprayed on the whole assembly, coating batteries and all of the interior surfaces of the pack. This leaves the metal shell of the batteries still connected to the active cooling elements, while the rest are coated. Any thermal runaway is thus shunted directly into the cooling system, while every other surface is protected by 4 layers of intumescent material (the two touching the battery, and the two on any opposing surface). And that isn't counting the intumescent layers that might be inside of the battery as well.
It seems simple, cheap and entirely fireproof, considering the small size of each individual battery. I am extremely impressed with the way that Tesla is using simple chemical reactions to snuff out thermal events before they occur, which then also create a mechanical barrier to reactions that still manage to get out of hand. This is aside from all of the active controls and safety features built into the macro-pack, including other mechanical barriers, cooling systems and power electronics.
In addition, two related pieces of IP which might or might not be implemented -
Battery Cell with a Center Pin Comprised of a Low Melting Point Material - Patent application
Battery Cell with Center Pin Comprised of an Intumescent Material - Patent application
In these patents, the stock central core is replaced with a core that either melts (to deform inwards) and/or contains an intumescent material. In a stock battery the configuration of the central core makes it more likely that the battery will burst during a thermal event. These patents cause the material to be sucked inwards, and/or helps cool down the reaction in the first place. . . .
[He then updated the post in 2/2014, after the first fires due to underbody penetration by road debris, which led to Tesla adding a shield, but before other fires that didn't involve such damage] Since this post/thread has been referenced a number of times in the media, I just want to clarify a few points that are important in hindsight.
First, its not certain that intumescent materials are being used in the Model S pack. In the case of the pack they built for Toyota, RAV4, they do not appear to have used an external application directly on the cells (we do not know if they coated other surfaces, or the interior of the batteries).
That said, the fire events that have occurred after striking debris seem to point to there being a significant delay between the initial impact and a serious fire breaking out. This would be entirely consistent with intumescents slowing the reaction, just as described in the patents.
Second, all of the analysis in this post about the unlikelihood of a fire in the Model S pack was based on the notion of a spontaneous fire cascading through the pack after the (possibly) spontaneous failure of a single cell. Ramming a heavy piece of metal through the armor shield would obviously disrupt a sizable number of cells, and none of the features described here could do more than slow or mitigate the resulting fire.
At that point the metal barriers that segment the pack and the capability of the pack to vent the fire into the frame of the vehicle (which then shuttles the flames to the front of the car to keep the passenger compartment and access points safe) become the key features that maintain passenger safety. The ability of the pack to vent the heat away from the passengers was a post I always intended to write up, but never got around to. In retrospect, it is clear that these features (at a minimum) work as designed in the real world.
I think it's fair to say that the claims made in the above post about how fires and thermal runaway will be prevented by Tesla's design changes and make them safer than stock batteries have proven to be over-optimistic, assuming that any of the patent ideas described were actually incorporated in the batteries.