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Comment by castratikron | Hacker News Reader

Why not use resistive braking to warm up the battery pack?

This is how locomotives implement braking; the drive motor operates in regen and the current goes to massive grid resistors. There are no brake pads.

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There are brake pads in a train, each car has them. I'm assume the engine does too but I'm not an expert. You are correct that the resistive braking is preferred - the energy to stop a train would eat brake pads if that is all they used. (I suspect air brakes don't allow as much control - but again here I'm outside my expertise)

The Model S and X have a dedicated battery warmer, the Model 3 does not as a cost cutting measure (it uses the motor windings to heat the coolant loop, but it appears this solution, while clever, is inadequate).

Why not resistively heat that cooling loop? Or release it to the cabin? Turn on the de-frost?

I kinda wish my ICE car would crank everything up while engine-braking down a hill to reduce brake and fuel use.

It shouldn’t be that expensive...

that's what the model 3 does. it induces resistance in the motor at 0 rpm to generate heat which is then transferred to the coolant loop to heat the battery.

but in order to warm up the very large battery pack, on a cold morning you need to turn the car on (well) before you start driving to have it warm enough for full regen.

You can, of course, do this from the Tesla app. It’s not like an ICE car where (usually) you have to physically be in it to start it.

With my ice car in the summer I often only turn on the AC only while going downhill or exiting the freeway.

I just wish it was more automatic. Hard to say if I'm saving pennies or dollars, but a $1 chip to do it should pay for itself quickly.

Just about every modern car cuts fuel when you're engine braking.

An modern (Last 20 years) ICE engine shouldn't use any fuel while engine braking. The ECU should just cut the fuel to the engine and allow it to run under vacuum.

My 15 year old Golf does and engine braking is why I've still got the original brake pads after 180K miles.

I’m not so sure this is how locomotives brake. After living beside a train track for 4 years I can tell you the cars are not using regenerative breaking to slow down. They use air brakes that are audible due to the brake pads and air.

https://en.wikipedia.org/wiki/Dynamic_braking#Rheostatic_bra...

This specifically addresses the air brakes you're hearing:

>Dynamic braking alone is insufficient to stop a locomotive, as its braking effect rapidly diminishes below about 10 to 12 miles per hour (16 to 19 km/h). Therefore, it is always used in conjunction with the regular air brake. This combined system is called blended braking.

That's improved with AC locomotives. Regenerative braking now works down to zero speed, if desired.[1] Locomotive motors today are three-phase variable frequency synchronous motors, run as servomotors. Like other servomotors, they can be controlled all the way down to a stop. At some point the friction brakes are applied, but that's usually done only at slower speeds.

Large modern locomotives often have "wings" with ventilation slots and fans. That's where the heat from the braking resistors gets dumped. This is a big win when moving heavy freight down long mountain grades. Friction brakes tend to overheat, but fan-cooled resistor banks can dump heat all day.

It's amazing that semiconductors for that kind of power not only exist, but are not that big.

[1] http://www.republiclocomotive.com/ac-traction-vs-dc-traction...

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