Another regen question

Regen braking in a passenger vehicle goes agaiinst just about everything you learned as a kid when you had your permit. Traditionally, in a combustion engine vehicle, drivers slow down by either coasting or downshifting. Applying the physical brakes is ideally only done when the vehcicle is near zero speed in order to ensure a full stop. This is all because of the dynamics of the engine. When the accelerator is not depressed, the combusion engine simply slows down until the driver wishes to re-accelerate. The inertia of the vehicle continues to carry it forward with a gradual deceleration based on speed, incline, etc. Other than the traction of the ground, there is no opposing force unless the brakes are applied.

In an electric motor, the energy always has to go somewhere. Electric motors do not allow vehicles to truly coast. When release the accelerator on an EV or a hybrid, the opposing force of the motor against the direction of travel produces regen braking, i.e. current is now flowing away from the motor instead of into it. This current is then directed back to the high-voltage batteries and in essence charges them to some degree.

On the Prius (II), the braking system has been improved over the series I in that regen braking is the force applied along the full braking curve just up to near-zero speed, where the mechanical brakes are then applied. On a series I, the mechanical brakes were allowed to engage much sooner. There is no harm nor brake wear by maximizing your regen braking. Therefore, the driver can actually ride the brake pedal when preparing for a stop or going down a long hill which would not normally be recommended in a combustion engine vehicle.

The direction of current flow in one direction or the other creates either an accelerating or decelerating force with no true coast in between. The closest approximation of "coasting" is to only gently apply the pedal to the point that the battery voltage applied to the motor causes the motor to move in the direction of travel with a minimum of resistance.

The inherent benefit of regen braking on a passenger vehicle is extremely precise speed (cruise) control. The closed loop nature of the current request and the current feedback can ensure a very constant vehicle speed regardless of inclination.

Enough with the engineering-speak (I couldn't help myself). The short answer is, as always, drive the way that's comfortable for you. There is no truly "better" way to drive. The data only shows that there isn't really a "bad" way to drive that would result in a shorter life of any of the major components.

 

That is very interesting

I have a question.
When I am coasting down a hill I see a charge going to the battery (of course engine off). So if I slightly push the brake if that's right will I see or get more recharge towards the battery?


Oh, yes I had a Camry Le V6 before the Prius and gas mileage of course is way higher also I notice a much stiffer (sportier) ride than the camry.

Thanks,
Bryan

 

I have to disagree with one of Mark's comments. Downshifting to an almost stop, especially in a manual, will cause a lot more wear on the transmission which is a lot more expensive to repair than the brakes. A coasting ICE car will slow down due to tire friction, transmission friction and engine compression. There is no exact equivilent on a flat road to Prius coasting shy of putting it in neutral.

The concept of coasting, from the physics standpoint, means you are causing the least amount of drag and therefore the least amount of loss. Similar to putting it in neutral, no gas or electricity used. Regen is not 100%. In a "get the best mpg possible" world, you would be able to anticipate every signal or stop and use the least amount of power to get there, preferably never slowing down for a signal and stopping only once for a stop sign after coasting to 8 mph where the friction brakes kick in. If you regen brake to the stop, you are putting some power back in the battery, but you used power to get up to that speed and are recovering only some of it.
Now to the REAL world. No one wants to coast to a stop. There is traffic behind you and besides, you wouldn't want to put up with the length of time it takes to coast to a stop. So you balance the two. Get it into coast mode when you can, on the flat. Regen brake to a stop, starting closer to the stop rather than all the way in, when the coast will be too long.

 

braking will create more regen, but to maximize efficiency, not braking will always be better.

regen captures only a percentage of the power needed to get up to your cruising speed. No matter how you stop, you are losing power even without using the friction brakes.

the reason is the power needed to regain speed will always be greater. so the highest average speed that averages below a certain level (probably close to the max EV speed of 42 mph) will yield the greatest mpg.

as a rule of thumb, if the speed limit on the street you are driving on is 35 mph, then averaging as close to that as possible without doing more than about 40 mph at any time will give you the best results.

the reason is simple. it takes up to 50% of your maximum torque just for moderate acceleration. the average driver uses as much as 70% from a standing start when driving a 4 cylinder car. however, to maintain 65 mph, only 8-15% of the engine's torque is necessary. so at 35 mph, it will be even lower.

so coasting is best and if you got the patience and you are alone on the road, you may find that astronomical mpg figures are available with the Prius.

you will also notice that in a regular car, nothing you do as far as driving will make a noticable difference.

i guess that is why i love the Prius so much. you get out of it what you put into it. increases of 25% in mpg's is normal by changing your driving habits. now to be honest with ya, i have never put forth the effort to get better mileage in a regular car, but i doubt that i would see this kind of results. i have taken more than enough long car trips in my life ( i have lived in or visited 38 states) to know that all freeway driving only gives me marginal increases in mpg and i took a lot of trips when the speed limit was 55.

 

60 amps of brake recharging of the battery is very difficult to attain. 57 amps a piece of cake, it happens just before the friction brakes start to cut in. And you have to be going about 40 miles per hour. When the speed drops to the 20 MPH range 20-25 amps tops. At 10 miles per hour 2-4 amps. I doubt that most would recognise when one is occuring and the other just starting. With the mini scanner just a glance and you know.

 

Here are my measurements on this topic :
http://www.vassfamily.net/ToyotaPrius/CAN/...hargeindex.html

Attila

 

Attila is this DC amps to the battery data or AC amps MG to inverter data?

 

Inquiring minds want to know.

Attila-
How many amps do you get when coasting at various speeds and terrain?

 

There was a posting in the Yahoo Prius group about a year ago by a very smart man named Wayne in which he measured regen efficiency, and his conclusion was that the highest efficiency was achieved when doing very light braking.

So that is what I always try to do when I'm slowing down. Just a light touch on the brake, while allowing myself plenty of distance to slow down.

P.S. My current tank's average is 55.9 MPG, with 101 miles on the tank.

 

Probably due to the level at which the battery can charge without wasting energy as heat due to increased resistance at higher charge rates.

 

Is everyone saying that the regen current is independent of the force with which you depress the brake pedal? In other words, if I decelerate from 60 mph to a dead stop the initial charging current is the same whether I let it "coast" (foot of the accelerator) or apply the brakes with the pedal???

Any link where I can read up in more detail on the subject?

 

This link should help:
http://www.vassfamily.net/ToyotaPrius/CAN/brindex.html

I can't make out any useful information on the other link.

 

<div class='quotetop'>QUOTE(DanMan32\";p=\"90293)</div>

Thanks, DanMan. Do you know what "little onboard computer" he is referring to?

 

<div class='quotetop'>QUOTE(rwlade\";p=\"90485)</div>

Thanks, DanMan. Do you know what "little onboard computer" he is referring to?

Never mind. I found the answer at http://www.vassfamily.net/ToyotaPrius/CAN/cindex.html
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Once the USB version of the dongle comes out, I am going to splurge and get it. Then I will try and get Attila's code to run under XWindows so the screen resolution is not forced.

 

I often drive in the Colorado mountains and see full (and empty) batteries quite frequently. Based on Attila's data, it seems that I should I use B-mode once the battery is full. It looks like very little extra charge goes into the battery at that point, so that means I would just be using my brake pads anyway, right?
Another B mode question. Does B mode work differently a low and high speeds in terms of engine braking vs. extra regeneration? At high speeds in B mode, it seems that the engine braking is used heavily, but what about at low speeds? Does B mainly just increase regen braking, or is the engine still responsible for most of the braking?
Until recently, I pretty much avoided B mode so as not to waste any possible regen energy. Now I'm starting to think it should be used more often, since hard braking uses friction brakes anyway. Could someone (like Attila) do some measurements in B mode to see if it maximizes regen current automatically and only uses engine braking for the braking that would be done by the brake pads anyway.