Using the Brake

There are numerous threads on the subject. Use "search".
.
Both regeneration and physical braking are applied over a smooth
continuum whose parameters are dictated by many inputs. You can
definitely learn to play this for best results.
.
_H*

 

Hi Montego,

We have been having this discussion on PriusOnline. Check out http://www.priusonline.com/viewtopic.php?t...bd5c533fe16695b .

For short, using Attila's data at http://www.vassfamily.net/ToyotaPrius/CAN/eveffindex it seems that about 81.5 percent of the motion energy is regenerated into the battery. And about 66.6 percent of battery energy gets converted into energy of motion for an acceleration. So, this works out to 54.3 percent times the battery losses. A lowball value for the battery efficiency is about 85 percent (I read someplace that 90 percent is more typical), and at that value a conservative total efficiency of reuse estimate is then about 46 percent. This is only one data point (from and to 50 km/hr - 31 mph) however. Actual driving conditions will be more variable.

The amount of energy regenerated is variable too. Check out http://www.vassfamily.net/ToyotaPrius/CAN/brindex.html for more details.

 

<div class='quotetop'>QUOTE(donee @ Jun 27 2006, 10:06 PM) [snapback]277900[/snapback]</div>

This seems reasonable, based on the amperage numbers I see on CAN-View when braking. The harder I press on the pedal, the higher the amperage. Pressing fairly hard, it's easy to see around 100 amps. Just letting off the accelerator, where the car emulates an automatic transmission by applying a small amount of regenerative braking, seems to generally produce around 15 or 20 amps.

 

Daniel I'm really in the dark on this subject. From your post it seems like your experience with CAN is that just letting off the gas gives a little re-gen. Then I think I get from your post the harder you brake the more re-gen you get. Question: do they combine? That is is 1+2=3? or does 2 cancel out 1 thereby=2? Next question is how does selecting B fit into the equasion according to CAN?

 

<div class='quotetop'>QUOTE(daniel @ Jul 2 2006, 08:26 AM) [snapback]280079[/snapback]</div>

That is in line with what I've read. However, 100 amps is max; based on bat. temp and other factors, it might max out at 80 amps. Any braking past the charge max uses the mechanical brakes, which means that energy is wasted as heat. You're better off braking at 50 amps over twice the time as your 100 amp stops, or risk wasting energy. Of course, that requires looking further ahead on the road, which is a good mileage trick in any car.

///d@

 

<div class='quotetop'>QUOTE(MarinJohn @ Jul 2 2006, 11:40 AM) [snapback]280100[/snapback]</div>

Yes, the regen from letting off the gas is the same as pressing the brake. Let off the gas, get a little regen. Press lightly on the brake and get a little more regen. Push harder and you get more, up to the limit of the MG sets and the battery. The brake peddle is also rate sensitive: if you mash on the brakes, the computer figures you are panic stopping and worries less about regen and more about stopping in a hurry. The friction brakes are applied early in a panic stop, instead of waiting for around 7 mph.

B mode shifts the engine into a high drag mode for engine braking, like downshifting for a big downhill. The rest of the braking system still works the same.

Tom

 

Regenning at very high currents is also likely to cause more ohmic
heating in the battery and wiring, thus more loss. Parts get warmer,
resistance increases, more and more losses.
.
_H*

 

Thanks Tom.

 

John: Sorry. I don't often open the Tech Discussion forum. But I agree with all three of the replies above.

I have seen it go just over 100 amps, but I think it likely, as Dithermaster says, that the actual max probably depends on various factors, including battery temp. A lot seems to depend on battery temp.

I don't generally stomp on the brakes. As hobbit says, it's probably less efficient. But I'm not a high-mpg guru either. And I've had my share of sudden stops. Doing 45 on a main drag and a light turns yellow just as you're close upon it, and it can take a 100-amp deceleration to stop. I'd rather waste a wee bit of energy than go through a yellow light if I can safely stop. Safety first. Then conservation.

 

B-mode brake or foot brake???

From quote above: "B mode shifts the engine into a high drag mode for engine braking, like downshifting for a big downhill. The rest of the braking system still works the same."

Newbie Question: When going downhill during and in a glide (in trying to keep under 41 MPH) session what braking method is best "B Mode/using the engine" or foot brake? I tried searching, but I have have not nailed an answer.

Thanks in advance.

 

Re: B-mode brake or foot brake???

Use the foot brake. The only time you ever need or want to use B mode is when a descent is so long and steep that you will max out the charge of the HV battery and still have more braking to do. This seldom happens unless you drive in the mountains. Using B mode throws away energy to forestall the use of the friction brakes, which happens when the regenerative system runs out of room for energy storage in the HV battery.

Tom

 

Re: B-mode brake or foot brake???

Thanks Tom! That is where my mind and foot (pun intended) went, but I was looking for confirmation.

 

Fuel economy principles:

- Accelerate briskly. The engine is most efficient under substantial load.
- Gliding (no arrows on the energy display) is better than coasting.
- Coasting is better than braking.
- Braking is better than stopping.
- Stopping is better than crashing :_>
- Never use "B", except to avoid riding the brake (for example, when coming down a really long slope)

 

- Never use "B", except to avoid riding the brake (for example, when coming down a really long slope)


Yeah, I saw that and laughed, dolp!

:jaw:

 

I figured riding the brake in the prius going downhill was actually not much of an issue unless the following occured:
1) Your battery is at 100% and therefore regen braking is "turned off".
or
2) You're exceeding the 100amp max

In AZ we have several mile steep downhill stretches. I've only used B mode a couple of times towards the end of a decent because my battery was at 100% (or I suppose 80% technically, which is 100% according to the MFD).

 

Riding the brake is not a problem, and should be preferentially used over B mode. Once again (I must have posted this same comment twenty times), B mode is used to throw away energy. Riding the brake down a hill takes the braking energy and saves as much as possible for later reuse. B mode takes some of the energy and dissipates it as heat through engine braking. This is a good idea if you are going to max out the charge on the HV battery, otherwise it is counter productive.

If the poster has said: "B mode is used to avoid riding the friction brakes when coming down a long, steep hill" I would agree completely.

Tom

 

One of the issues is that riding the brake can easily result in riding the friction brake. Frequently when braking while passing over a bump, pothole, or large crack in the road regenerative braking drops out and the vehicle engages friction braking. Regenerative braking is not restored until I've fully removed my foot from the brake pedal for some amount of time. For short hills or other typical stopping, the amount of time friction brakes have been engaged in this way is insignificant in comparison to the energy that would be wasted if I always used "B" mode when braking. However, when driving down a long hill steep enough to need to ride the brakes the whole way, if a bump in the road activated the friction brake at near the top of the hill being in "B" mode would allow removing my foot from the brake long enough to re-engage regenerative braking without unnecessary acceleration.

For this reason I expect I'll switch to "B" mode anytime I'm on a long enough hill to expect to have a fully (80%) charged battery at the bottom.

Riding the brake without first engaging "B" on such a hill would, at best, simply fill the battery and then engage the ICE in a fashion similar to "B" mode, throwing away the additional energy anyhow, and at worst could result in brake fade and a battery that isn't fully charged, which would be far less efficient than "B" mode.

 

Right. The key is knowing how much hill you need before B mode becomes useful. This will vary depending on the initial SOC of the battery and a few minor factors such as temperature. Where we live there are no hills that ever benefit from the use of B mode, so the choice is simple. If you drive in the mountains it's a different story. Only experience will tell you when to use it for maximum efficiency, but from a safety standpoint you can never go wrong by using B mode. If I were heading down a long slope and had no prior knowledge of the grade and length, I would switch to B mode at the top just to be safe.

Tom

 

In over 4 years of mountain driving I have yet to use B mode.

With all of the electronics Toyota built into this car to manage SOC I just don't have time to worry about it.

I have never experienced the loss of regenerative braking due to road conditions. However I tend to stay on hard surface roads.