Prius Regen Braking

toyota
prius

#1

Hi Folks,

I have a 2006 Prius that I love. Over 200k, going strong, with hardly any repairs other than routine maintenance. Pretty cool!

I had a question re: something I’ve always wondered about.

Let’s say you accelerate, then hit the brakes. Does anyone know the percentage of power that’s returned to the batteries from the regenerative brake? Is it meaningful, or just in the single digits, percent-wise?

Any leads to any info on this? Thanks. It’ll help settle a long-standing office discussion! :slight_smile:

Kilian


#2

It can be quite substantial if you brake gently, with most all the braking coming from the regen. I can watch my battery recharge as I do that. As for a percent, I don’t know. You might ask over at Priuschat to see if somebody there has some data on it. The regen is a major reason the Prius gets such good in town mpgs.


#3

It’s significant.

On my mom’s Fusion hybrid (technology leased from Toyota), you can recharge the battery at least halfway just measured descending of a 800’ hill, provided you brake gently and don’t exceed 42 MPH.

I doubt the recapture is more than 50% efficient…but I also doubt it’s a whole lot less. Remember, though, that energy cannot be created or destroyed–just converted from one form to another, and no conversion process is ever 100% efficient.

OTOH, mechanical brakes are 0% efficient–they turn motion into heat that serves no purpose, and is a PITA to get rid of!


#4

The system will try to capture all the energy that it can and return it to the HV battery…The best results are obtained by applying the brakes gently then slowly increasing braking effort…You can feel it happen…At a certain point as you increase pedal pressure, the standard friction brakes are applied…But with careful driving, the friction brakes on your Prius should last forever…In the Lexus CT200h, a Prius with a different body style, there is a dashboard display that shows the regenerative charging level as you apply the brakes…With the meter showing maximum charging, the car is decelerating strongly without the friction brakes being engaged.

So I would say that with careful driving, most of the kinetic energy stored in the moving car is returned to the battery when you brake…


#5

Thanks for this, guys! I learned a lot-- time to take this to work to see if it settles the debate!

Great forum here!

KJ


#6

If they don’t believe you, ask them to explain why hybrids get such great mpgs in town.


#7

Not only that, but I got as good mpg driving up and down the mountains of West Virginia as I got on level ground in a recent trip. That regen is amazing.


#8

Yep that ReGen technology appeals to the Scot and Norweigen in me,thats why they get such good mileage in town,the manus could easily turn out a car that got similar highway mpgs(but they wont) and such a car would fall short in city driving-Kevin


#9

I’m not a Prius specialist but there are a few that I service regularly. I have one customer that has over 200,000 miles on the original brakes and they still have over 30% pad remaining. This in stop and go driving in the Seattle area where it’s normal to wear out a set of brakes in 50-60,000 miles.

So just assuming that the brakes on the Prius will last 5 times as long as the brakes on a similar sized car I would imagine that the amount of regen is quite substantial.

I can see why taxicabs are switching to Priuses instead of Crown Victorias. The savings in brakes alone would be substantial.


#10

To make a living with a cab, the vehicle has to be economical to operate and also be reliable…I guess the Prius meets those requirements…


#11

Taxi cabs are PERFECT for a hybrid.

Go to NYC or Boston and you’ll see Camry Hybrid taxi’s all over the place. Yes the initial up front cost is more…but those cars see a lot of PERFECT HYBRID MILES. So the payback will be 1-2 years as opposed to 4-5 years.


#12

I’ve read that the efficiency is approximately 30%. Sorry, I can’t quote the source information, as I don’t remember where to info originated (it was from the Prius Chat site that houses a lot of engineers who love such knowledge). So while you still lose 70% of the energy it took to get your car up to speed when braking, at least you get some energy back (mechanical brakes = 0% regeneration).


#13

To expand on texases’ comment, if you touch the brake to get regen but no mechanical braking, you are recovering about 90% of the kinetic energy of the car (basically the efficiency of the charging system). If you slam on the brakes, you get maybe 5%. Average probably is in the 30-50% range as noted.


#14

The engineer in me wants numbers; Assume 70% front braking on the FWD Prius, assume a small friction brake effort of 20% so the regen is 0.7x0.8=0.56. So 56% of the overall braking is regen. There are losses through the motor itself, the wiring and the controls, say 15% max and the battery is not 100% efficient on re-charge, say 3% loss that leaves 46% of the regen energy gets into the battery. Pretty good. The Prius will also lose some energy going back to the wheels so that same 15% and 3% losses occur going TO the wheels so 38% gets back to the tire contact patch to move the car.

Electric drive systems are pretty efficient but you have losses in all 3 pieces. A 93% efficient A/C motor through a 92% efficient controller with a 3% loss at the battery (they get hot, you know) leaves 83% of the energy to power the car. Efficient, yes, but not as high as published because most ignore the controller and battery.


#15

Ford paid Toyota for hybrid technology that they ripped off? That’s rich.


#16

I believe in NYC you either had to use the official NYC cab, formerly a stretched Crown Vic, soon to be a Nissan SUV, or you had the option in recent years of using an approved hybrid. Guess which is not just much cheaper to buy, but also much cheaper to operate? San Francisco has been requiring that all cabs be hybrids for a few years. I bet Boston has a law, too. In practice a hybrid is perfect for cab use. Excellent city gas mileage, and the engine and brakes are less stressed than non-hybrids.

When the NYC fleets started experimenting with them a few years ago they were amazed at how long the brakes lasted. They also found the hybrid drivers were getting in fewer accidents. My guess is the old Crown Vics made the drivers feel more protected, and they were more likely to cut off other drivers and engage in other hazardous behavior. I hate the cabbies there, but I like them in San Francisco. It’s like they give potential cabbies a personality test and send all the belligerent, competitive, rude drivers to NYC and the pleasant, laid back ones to SF. Many of the cabbies here are middle-aged and older white guys who listen to jazz or classical and have excellent driving skills. They’re a chatty bunch. The rest are the stereotypical recent immigrants, but even they mostly drive well and are polite. It’s nice not having to worry you’ll end up in a road rage incident. I once had a three-block NYC cab ride (I had luggage) and in those three blocks my driver and another cabbie kept trying to cut each other off at signals and were screaming at each other by Penn Station. Guess who didn’t get a tip?