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Comment by mikestew | Hacker News Reader

What about the brake pads? With regenerative braking, I expect the pads on my Nissan Leaf to last almost as long as the car.

Whether the $600/year is a good price is irrelevant. The matter at hand is "pay us $600/year or we void your warranty". It's shady, for one, and of questionable legality given Magnuson-Moss.

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Was going to say what your parent said, but now I realize I might be missing key info about regenerative braking.

Do these types of cars use the engine for most braking, even when the pedal is pressed?

It would make sense. The pedal is probably electronic anyway, so the car would try to use engine for regenerative breaking, and use pads only when the break pedal is severely pressed.

With the regenerative braking on the Nissan Leaf (what I own and can speak to with some authority), pressing the brake pedal does not mean the pads touch disk. What happens most of the time, and for most of the braking right up until the car comes to a stop, is that the electric motor is turned into an electric generator. I can watch the meter on the dashboard switch from sucking-juice-from-battery to dumping-juice-in when I hit the brakes. I also assume there are some large capacitors involved, as I can't imagine any battery likes having 30kW being dumped into it.

As far as I can tell the only time pads touch disk are 1) emergency stops when the generator can't cause enough load to safely stop the vehicle 2) below about 10mph (not much load from the generator) and 3) the battery is fully charged and has no room for additional electrons. The transition from generator to pad-and-disk can be felt if one is looking for it.

In summary, the vast majority of the braking on the Nissan Leaf appears to be done without the conventional brakes. I assume the Tesla works in a similar manner, hence my comment.

EDIT: added a third reason for conventional brake use.

The Nissan Leaf is designed for short trips and city driving primarily. In those conditions a significant amount of braking is done under 10 miles an hour.

Also, in almost all 2 wheel drive cars, regenerative braking only applies to the wheels being driven and standard, dynamic braking is applied to the non-powered wheels.

I would sincerely be amazed if your brake pads lasted you even half the lifetime of the car and I will willingly hold my hands up and admit defeat if I'm wrong.

I can't comment on how long brake life is, but the Nissan Leaf is meant to be used like how most people use their existing cars, around town driving. There are plenty of 35, 40, 45, 55+ mph zones throughout most cities. People are only braking from 10 mph when going through a parking lot. I doubt a significant amount of Leaf drivers are driving through parking lots all day long. If you're suggesting that freeways turn into parking lots during rush hours, then your statement could be said for all cars not just the Nissan Leaf.

IANA Automotive Engineer, but I notice that while much breaking happens below 10mph (everytime you come to a stop from any speed, you're doing some breaking under 10mph, right?) break pads are likely to get worn far more at higher speeds - kinetic energy depends on the square of the speed, right? So decelerating from 35mph to 10mph, the break pads need to absorb (for some k) k * 35^2 - k * 10^2 units of energy, while decelerating from 10mph to 0mph is k * 10^2. The difference is more than an order of magnitude, and it only gets bigger as the speed increases. This doesn't even take into account all the times we slow from 35mph to 25mph, which is about 6 times as much energy as slowing from 10mph to a stop.

I don't see how regenerative braking makes a difference? Correct me if I'm wrong but if the pad comes in contact with the brake disc every time you press the brakes, regardless of whether it generates energy or not, I don't see why the compound of the brake pad would wear down any differently to standard brakes.

Regenerative breaking doesn't use pads. It basically uses engine to slow down the wheels.

Sorry but that's just categorically wrong. The leaf, just like every new car on the planet uses disks & brakes. A quick google search of Nissan Leaf brake pads will bring up tons of results for aftermarket replacement pads.

What parent said is correct, categorically or not: regenerative braking does not use conventional pad-and-disk. It uses the electric motor to slow the car by turning the motor into a generator. The fact that a Nissan Leaf has pads and disks does not change this.

Take a look at this: http://en.wikipedia.org/wiki/Regenerative_brake

Yes, the leaf has standard brakes... but it also uses regenerative braking to recapture some of the car's forward momentum without using the brakes.

And any forward momentum captured this way is not absorbed by the brake pads, thus they last much longer.

This really isn't that complicated. Electric cars use a combination of regenerative braking (using the generator to convert kinetic energy; forward velocity, into electric energy) and using the disc brakes when regenerative braking doesn't slow you down fast enough. This results in much less wear on the brake pads and callipers.

What you are missing is that the Leaf is drive-by-wire -- pressing the pedal does NOT always activate the physical brakes. For light-to-medium braking, the electric motors can be operated in reverse to decelerate the vehicle.

More generally, the wear on the brake pads is roughly related to the amount of energy they soak up. Energy that gets pulled back into the battery by regenerative braking does not go towards wearing the brakes.

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