Comment by arghwhat
7 years ago
Note that trains are a very different scenario due to not being battery powered.
Many (most?) trains dump excess charge into the grid, meaning that it has no limitations on brake power nor capacity.
Battery-powered vehicles have to protect their battery, limiting brake power to keep the battery cool, and brake duration as it cannot overcharge the battery.
To compete with a train, a battery powered vehicle would need to implement rheostatic braking (i.e. brake resistors).
Of course, conventional engine braking is not worth comparing to any of this, but I thought it was important to emphasize that trains ≠ cars.
Of course there are many differences between a train and a car.
But the technology is exactly the same: apply an electrical load to a motor/generator and it will generate a braking force. The source of the load doesn’t really make a difference as long as it meets implementation requirements.
(Perhaps maybe the most ironic way they could dump excess electricity could be by running the onboard air compressor — literally the same thing an ICE vehicle does to dump energy when engine braking: compressing air.)
My point is that the technology is definitely a good fit for braking large loads.
Of course, except that normal cars have no component to dissipate the load. The battery is the only way, and it has significant limitations.
A resonable auxillary air compressor won't make a difference. With a heavy trailer going down hill, you'd need to at least dissipate tens of kilowatts, maybe even touching triple digits if you also need to slow down.
Nowhere to dissipate the power → no regenerative braking.
There’s probably a few things this truck has that normal cars don’t have.
The air compressor comment was an illustrative tougue-in-cheek comparison, not a serious suggestion.
Imagine the amount of power needed to run an air compressor with a displacement in the 5-6 liter range. That’s exactly the amount of power we’re talking about. Because that’s literally what an engine-braking truck is doing, it’s driving its engine as an air compressor.
Your “tens of thousands of watts” estimate is probably just about right. And that’s not anywhere outside the realm of doable. That’s probably well within the abilities of regenerative system in the drivetrain this vehicle will have, but even if we assume it isn’t, a 10kw resistive load is a $100-$200 part, off the shelf.
My PHEV minivan (Chrysler Pacifica) does close to 100 kW of regen in hard braking. I imagine the Cybertruck could easily double that, probably a lot more.
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