How would regenerative braking work for a long downhill?

[PandaBear]

I think the idea is that in some sections, it's really steep and there might be a low speed limit (like 10-25 MPH). I was recently driving on Pinehurst Road near Oakland, and there are some sections where it's steep with super tight turns. And the big thing would be a last ditch if the brakes fail. Damaging the engine might be preferable to losing control. I've personally gone down a hill in 1st gear. Especially Marin Ave. in Berkeley, which is really nasty. In my 1995 Integra GS-R, it didn't reach redline (8000 RPM) at my 20 MPH speed. My WRX doesn't reach redline in 1st at 25 MPH. It' might be around 5000 RPM and gets really loud, but I'm thinking it's not that bad if it's just compressing. At that point the fuel is going to be cut off anyways by the ECU.

I've also been present when someone clearly lost brakes (likely fluid boil) and I could hear the handbrake being applied. I talked to the driver, and she said once she lost it she pulled the handbrake and put it in 2nd, which was the lowest should could do. It slowed it down to about 30 MPH down the steep hill in front on my house where it finally stopped at this weird curb (hard to describe) and jumped over it. The occupants walked out but with the airbags deployed and with clear seatbelt bruises. I think 1st gear might have been preferable. On something like a long stretch, it might be enough time for the brake fluid to cool and the fade to go away.

I think you are correct that there are always some steep slope braking concerns, even for gas cars. However, most cars are designed to a certain performance and public roads are designed to a certain safety standard. You can never protect yourself for every malfunction and sometimes you just have to accept the fact that a car will be totaled (i.e. ice on road going down highway 17), and hopefully police would be there to close it till it is safe. That's why we have airbags and seatbelts.

Over-reving engine may cause the valve to hit the piston. I don't think most major roads would have problem with EV braking due to brake size reduction. Many low end gasoline cars have even smaller brakes and less brake cooling and they go downhill fine, and a lot of people don't engine brake going downhill.

If it is still a concern to you, maybe upgrading brake pads to higher performance version? There's always brake kits you know.

 

[JHZR2]

My accord hybrid will turn on the engine and engage it if the battery SOC is too high and it is regenerating. I suspect it just is compression braking, nothing special.

 

[Kiwi_ME]

That's kind of tricky since these systems tie in both regenerative braking and physical braking. But it's my understanding that there's an actual mechanical link between the brake pedal and the brake hydraulics in most EVs.

I suspect that's true for all EVs because they would otherwise risk their liability in a courtroom. Hyundai/Kia brake systems integrate all required systems into one package, which they call iMEB. With no 12V power applied the hydraulics default to a conventional non-assisted system just a conventional car.

When the traction battery has less than 100% charge and is able to accept regen power, when the driver applies the foot brake the iMEB uses a motor-driven piston pump to initially provide brake pedal feel and electrically-requests negative torque from the traction motor, what is commonly called "regen". Upon reaching walking speeds where regen is essentially zero, the motor-driven cylinder then provides conventional power assisted braking. The same pump does ABS and collision-avoidance braking functions to eliminate that separate traditional unit.

I’m borrowing a Model 3 today. Not sure why it’s at 100% as I thought I remotely changed it to 90%. But also set it to apply friction brakes. Saw something new today, with a gray bar showing application of friction brakes, including simultaneously with limited regenerative braking in green. But the warning symbol for limited regenerative braking didn’t pop up.

Tesla use an electrically-assisted master cylinder that cannot invoke regen. They rely on drivers using the one-pedal mode to discourage them from using the brake pedal and wasting energy. My understanding is that at low speeds the brakes are then electrically operated to hold the car in position.

I guess in the early EV's or hybrids with resistive heating you could slam on the heat to full and open the windows, for a small draw on the battery, but a 3-4klbs of car rolling down a steep grade would need a big big resistor to change to absorb excess regen. It would be far cheaper and easier to just put bigger brakes on it I think.

Unless you mean EVs from the early 1900s like a Baker Electric even GM's late-'90s EV1 had regenerative braking invoked by the master cylinder.

 

[PandaBear]

I suspect that's true for all EVs because they would otherwise risk their liability in a courtroom. Hyundai/Kia brake systems integrate all required systems into one package, which they call iMEB. With no 12V power applied the hydraulics default to a conventional non-assisted system just a conventional car.

When the traction battery has less than 100% charge and is able to accept regen power, when the driver applies the foot brake the iMEB uses a motor-driven piston pump to initially provide brake pedal feel and electrically-requests negative torque from the traction motor, what is commonly called "regen". Upon reaching walking speeds where regen is essentially zero, the motor-driven cylinder then provides conventional power assisted braking. The same pump does ABS and collision-avoidance braking functions to eliminate that separate traditional unit.

Tesla use an electrically-assisted master cylinder that cannot invoke regen. They rely on drivers using the one-pedal mode to discourage them from using the brake pedal and wasting energy. My understanding is that at low speeds the brakes are then electrically operated to hold the car in position.

Unless you mean EVs from the early 1900s like a Baker Electric even GM's late-'90s EV1 had regenerative braking invoked by the master cylinder.

You do not have to "waste energy" using brake pedal. Prius have been doing that and it works fine without 1 pedal driving. The only reason 1 pedal driving "saves energy" is driver tends to slow down early instead of coasting till much later and then brake harder. I can see hydraulic braking staying for quite a while until regen braking can handle emergency braking with sufficient internal battery backup, or passenger car moves to some sort of "air brake" like system commercial trucks use for emergency. Relying on regen alone means battery range has to reduce to accept a fully charged car going from 100-0 downhill to be fully stopped without damage to the system (battery mainly), that may not be any cheaper than keeping hydraulic in place just for liability (avoid potential massive recall) and marketing reason.

 

[Torrid]

You do not have to "waste energy" using brake pedal. Prius have been doing that and it works fine without 1 pedal driving. The only reason 1 pedal driving "saves energy" is driver tends to slow down early instead of coasting till much later and then brake harder. I can see hydraulic braking staying for quite a while until regen braking can handle emergency braking with sufficient internal battery backup, or passenger car moves to some sort of "air brake" like system commercial trucks use for emergency. Relying on regen alone means battery range has to reduce to accept a fully charged car going from 100-0 downhill to be fully stopped without damage to the system (battery mainly), that may not be any cheaper than keeping hydraulic in place just for liability (avoid potential massive recall) and marketing reason.

The only other way that regenerative braking would work at full charge would not actually be regenerative. It would be like dynamic braking. You'd need a resistor grid which would take up space and generate heat. It's just easier to to default to friction brakes at that point.

 

[Jetronic]

I don't think going on 1st is a good idea even for engine braking. As far as I know you probably can't drive on 1st for above 15 mph without reving it too high, and downshift to 1st for engine braking is really, really bad for the engine. I would say if you try to go that low you can risk over reving past redline if you are not careful. 2 really is the lowest you should go.

My understanding is, if you are not charging at the peak, you get up the hill by your battery power, and you will not overcharge your battery when you are going down if you are regen braking, and you are not going to overheat your cooling system more than you going up. The only exception is if your car is not designed right and it has smaller regen braking power circuit than the driving circuit. If you buy a reputable EV from a major company you should not have this problem.

My main concern would be whether the friction brake overheat in a short stretch of very steep slope, like grapevine on I5, with 6% grade and 1500 ft drop in 5 miles. If you see runaway truck lane like those on grapevine that when I would worry. I never had brake fade problem on my Prius coming back from Tahoe and the altitude is even higher, but over a much further distance.

1st gear is good until about 30 mph in my car, 2nd until 65 ish

 

[Jetronic]

You do not have to "waste energy" using brake pedal. Prius have been doing that and it works fine without 1 pedal driving. The only reason 1 pedal driving "saves energy" is driver tends to slow down early instead of coasting till much later and then brake harder. I can see hydraulic braking staying for quite a while until regen braking can handle emergency braking with sufficient internal battery backup, or passenger car moves to some sort of "air brake" like system commercial trucks use for emergency. Relying on regen alone means battery range has to reduce to accept a fully charged car going from 100-0 downhill to be fully stopped without damage to the system (battery mainly), that may not be any cheaper than keeping hydraulic in place just for liability (avoid potential massive recall) and marketing reason.

The busses I drive have regen braking. If the battery is full, the power is sent to a resistor pack in the coolant circuit. You could use it indefinitely. It can bring the bus to a crawl on a 12% downgrade..... but below 3 mph you lose the regen braking so friction brakes are always needed to come to a complete stop, and to hold the vehicle stationary

 

[dogememe]

I don't think going on 1st is a good idea even for engine braking. As far as I know you probably can't drive on 1st for above 15 mph without reving it too high, and downshift to 1st for engine braking is really, really bad for the engine. I would say if you try to go that low you can risk over reving past redline if you are not careful. 2 really is the lowest you should go.

My understanding is, if you are not charging at the peak, you get up the hill by your battery power, and you will not overcharge your battery when you are going down if you are regen braking, and you are not going to overheat your cooling system more than you going up. The only exception is if your car is not designed right and it has smaller regen braking power circuit than the driving circuit. If you buy a reputable EV from a major company you should not have this problem.

My main concern would be whether the friction brake overheat in a short stretch of very steep slope, like grapevine on I5, with 6% grade and 1500 ft drop in 5 miles. If you see runaway truck lane like those on grapevine that when I would worry. I never had brake fade problem on my Prius coming back from Tahoe and the altitude is even higher, but over a much further distance.

No issues with brakes on my Volt on the way down the grapevine. The engine was definitely making a lot of noise in “mountain mode” going up though and didn’t have enough power to sustain more than 75mph. As far as I know it uses the same brakes as a Chevy Cruze which is a lighter car. I did gain some battery back on the way down which was nice.

 

[Torrid]

The busses I drive have regen braking. If the battery is full, the power is sent to a resistor pack in the coolant circuit. You could use it indefinitely. It can bring the bus to a crawl on a 12% downgrade..... but below 3 mph you lose the regen braking so friction brakes are always needed to come to a complete stop, and to hold the vehicle stationary

That's what I was referring to. Interesting. I wasn't sure if there was a dual system in operation where it had a dynamic grid and a battery that can be charged. It's used in locomotives too, but it only has the resistive dynamic braking grid and no battery system to recover to since it's only operated directly from generator to traction motor.

Working clear until 3mph sounds like it's using what is equivalent to extended range dynamic braking. It's pretty efficient but falls off at very low speeds since it requires movement for it to generate any resistance.

 

[PandaBear]

That's what I was referring to. Interesting. I wasn't sure if there was a dual system in operation where it had a dynamic grid and a battery that can be charged. It's used in locomotives too, but it only has the resistive dynamic braking grid and no battery system to recover to since it's only operated directly from generator to traction motor.

Working clear until 3mph sounds like it's using what is equivalent to extended range dynamic braking. It's pretty efficient but falls off at very low speeds since it requires movement for it to generate any resistance.

It "may" still work below 3mph for cars but it is very hard to control the precision unlike friction brake. Most driving demand higher precision on acceleration than braking and there aren't much energy to wear out the brake pads or recharge the battery with anyways. As long as we kept the speed low there aren't much brake demand so a small brake should work very well.

 

[MSCH]

The busses I drive have regen braking. If the battery is full, the power is sent to a resistor pack in the coolant circuit. You could use it indefinitely. It can bring the bus to a crawl on a 12% downgrade..... but below 3 mph you lose the regen braking so friction brakes are always needed to come to a complete stop, and to hold the vehicle stationary

that is how locomotives do it... since they dont regen, they load a set of resistors when descending a grade