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Comment by Retric | Hacker News Reader

It’s likely fine, 65 kW is only ~85 HP. Simply maintaining highway speeds is ~25HP, so as far as the tire is concerned it’s the equivalent of mild acceleration or breaking.

The regenerative breaking system is likely designed for long mountain roads, so it might overheat but probably not.

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Teslas will switch to friction brakes when regeneration limits (peak current or battery SOC) are exceeded. I’ve sustained constant 50kw regen while downhill through Appalachia passes, no issues, with the caveat that regen current is limited if the pack is cold until it approaches operating temperature (yellow dashed lines on the regen current display indicate your reduced regen capability).

Teslas have no resistor banks?

A great omission.

Why would a resistor bank be better than mechanical brakes? Both get rid of excess power as heat, and you probably need the mechanical brakes anyway.

What is implied by the GP is that the cost of ownership for regenerative brakes is lower per joule.

That's obviously true when that joule is "reused" and not so obvious when it's dissipated. Mechanical brake pads have to be replaced but I assume there is wear on the (more expensive?) parts in a regen system too.

Electric braking has close to no wear, and much cheaper EVs use it.

A resistor bank with liquid cooling will be few kilogrammes at most.

> Why would a resistor bank be better than mechanical brakes?

Resistors don't wear out like frictional brakes do.

A resistor bank of the required wattage would simply be a big lump of dead weight most of the time. And it would need its own cooling fan which would add even more weight.

It wouldn't be, all teslas already have liquid cooling.

Few kilogrammes at most.

There's a free 1000+ lb steel heatsink - the chassis of the car.

It isn't worth the complexity for the rare cases when it might be useful. Even if you live at the top of a mountain, the easier solution is to just not charge to 100% to account for your "free" energy gains when leaving.

I don't think the brake pedal was being used in this test, though.

My understanding is that above 95% SOC, even if you’re not using the brake, regen will not occur and an error is presented to the user notifying them about regen limits.

Sure, so the car stops charging and still won't waste/damage its brakes in the tow.

Does this mean that when the battery is full you must use the brake pedal? Or does the software automatically apply the friction brakes when you back off the accelerator?

that's correct. You don't get one pedal braking if it's not charging.

Regen braking is noticeably less powerful when your battery is nearly full, and you have to manually use your friction brakes more. Sounds like it might be a safety issue, but in any scenario where you use regen brakes you have more than enough time to switch to physical brakes if you want to.

65 kW is a little over 87 horsepower (parent previously claimed ~80 HP, now edited to 85).

To get 65 kW into the battery, you have mechanical and electrical conversion losses on top of that. The tires are seeing pretty close to 100HP.

That really depends on what the 65 kW being displayed represents. If it’s the AC power from the regenerative breaks that should be extremely efficient, post AC/DC conversion things look worse etc.

The article claim is pretty clear:

> putting back electricity into the batter[y] at a rate of 65 kW

The article could be wrong, of course.

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