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Does anyone know what actually cought fire? Can batteries catch fire due to being struck like that? If so, why did it take 5 minutes and how did the car electronics know about it beforehand?

If you strike the cells, they can be short circuited, leading to heating. Lithium cells can experience thermal runaway [http://en.wikipedia.org/wiki/Thermal_runaway#Batteries] when they're failing, heating adjacent cells causing them to fail. This is why cells are compartmentalized. Think of them like properly engineering bulkheads on a ship. If certain compartments fail, others are safely isolated.

The problem is, as long as drivers demand range with electric vehicles, and energy storage is much less than liquid petroleum, the bottom of the vehicle is the only place that large, heavy pack arrangement can go. You make the tradeoff: How much armoring needs to take place to avoid the most common damage/failure scenarios.

I have no doubt Tesla is learning quickly from these events, and is plaining alternate or improved methods to armor the pack from road debris and damage.

A gas tank is on the bottom as well, though I guess it doesn't run the entire length of the car either.

A gas tank presents a smaller profile under the vehicle than the Model S battery back, which covers a substantial portion of the undercarriage.

It's also usually located behind the rear axle. Something which is shaped in a way as to lodge itself against the road and the bottom of the vehicle to it is driven upwards is likely going to have done this before it gets to the rear axle.

Most of the vehicles on the road are front wheel drive and thus don't have a rear axle. However, the gas tank is usually mounted slightly higher than the floorboard in front of it, so that offers some protection as well.

The other thing to keep in mind is that the fuel tank is usually located in the rear of the vehicle while potential ignition sources (engine) is located in the front. This can reduce the risk of a fire as well. However, the exhaust system runs to the back of the car.

I am pretty sure the cells are compartmentalized so I would assume what happened was a slow process of each compartment heating up to a threshold eventually causing a chain reaction.

Aren't batteries under the cabin floor though? That hot-swap demo from few months ago seemed to imply that they were.

Yes, they are but I am saying the cells are in their own mini compartments. Think of like a ship has compartments to keep the entire thing from flooding if it is breached. At least that is how I have understood it to be. Obviously this doesn't prevent heat from spreading and could still have a runaway chain reaction like in this case but it does maybe give more time to warn you.

They are, but they have heat vents that spew off into the front, to protect the cabin itself. That is why the front of the car caught fire.

The battery itself is also segmented such that thermal runaway doesn't cause a cascade to the whole battery (which is why the driver here had ample time to continue driving and pull over).

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