Another regenerating braking question

<div class='quotetop'>QUOTE(Sho-Bud @ Jul 5 2006, 08:33 AM) [snapback]281415[/snapback]</div>

Rear brakes do not come into play with regenerative braking. The car is front wheel drive and the MG's do all of the regenerative braking. In emergency braking or under 9 MPH both front and rear brakes function as on any other car.

 

Actually if you exceed regen capability, the rear brakes are brought
in *first* to add in more braking power -- makes sense, to balance
against the front wheels which are already resisting 20 kW of decel.
.
_H*

 

<div class='quotetop'>QUOTE(seasalsa @ Jul 5 2006, 09:44 AM) [snapback]281473[/snapback]</div>

I'm curious where the "under 9 mph both front and read brakes function as any other car" factoid comes from. Please understand I am not questioning anyone's integrity, I just wanted to find this out and apparently I missed it in the owner's manual.

I have a steep hill near my home and have tried to figure out the best way to maximize the charging of the battery while going down the hill. For safety some amount of braking needs to be done.

Armed with this information, I will try keeping my speed above 9mph and feathering the brake petal.

 

<div class='quotetop'>QUOTE(whodat @ Jul 20 2006, 11:18 PM) [snapback]289781[/snapback]</div>

It's actually quite noticable (well, maybe not that noticable) when it switches from regen brakes to friction brakes. Although, I definately notice it at 7 MPH, so maybe '06s are slightly different than my '05. But, next time you're coming to a stop with the brake pedal gently depressed, you should be able to feel a slight change in the motion of the car when you pass the speed threshold. It's subtle, but it's definately there. I didn't know that the rear brakes came in first below the speed threshold (whatever that threshold is), although I certainly have no reason to doubt it.

 

On my classic Prius you could clearly feel when the friction brakes were added. You often had to let up a little on the brake pedal. On my '04 I can not feel it at all.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 20 2006, 08:27 PM) [snapback]289786[/snapback]</div>

Maybe I need to go to sensitivity training. Prius sensitivity training. I suppose CAN VIEW would be the ultimate answer, but my 2006 Prius will have to wait until our good friend Eflier gets enough interest from 2006/2007 Prius drivers.

In the mean time I will try to feel the force.

Dave

<div class='quotetop'>QUOTE(efusco @ Jul 20 2006, 08:58 PM) [snapback]289797[/snapback]</div>

This makes sense from an physics perspective. The regenerative braking is a inductive circuit with a static magnet and a rotating magnet. Once the angular velocity drops below some threshold, the amount of back EMF force that can be generated is substantially reduced.

 

<div class='quotetop'>QUOTE(whodat @ Jul 20 2006, 08:18 PM) [snapback]289781[/snapback]</div>

You do understand, I hope, that pushing the brake pedal when moving at over a few MPH, in other than a hard panic stop, always uses regenrative braking. For safety sake, just step on the brake. You'll get a lot more regen braking than by just taking your foot off the accelerator pedal.

 

Hi folks, I am resurrecting a dead horse... One thing I haven't quite understand: 04+ Prius is said to be able to regeneration brake at over 7 mph and I have noticed it. Online, some people said that you should brake first lightly as at high speed, the generator out-rush current is too much for battery to absorb quickly; and then when speed lowers, you should brake harder, presumably raise the "gear ratio" so that the generator will turn at the same rpm, and output enough voltage to charge the battery.

This makes sense for a traditional self-excited motor/generator (Prius uses a step or synchronous motor if I am right); however, I could not find any gear ratio adjustable between wheel and MG to speed up MG rpm at low vehicle speed (does the engine somehow come into play? But that is MG1 only); second, at low speed, the MG output voltage will indeed be very low, to charge the battery, it will have to be boosted. Is there such as mechanism to boost the voltage? So that at low speed the battery can still be charged, and MG has higher regenerative current running on it to provide enough resistive force?

Third, the MG1 and MG2 both use permanent magnetic on its rotor to reduce energy and maintenance. Are they at risk of demagnetization after a long time, say 10 years?

 

Hi Gus. Since I was the last to :deadhorse: I guess I'll take a swing at this too.

First, the electric motors are running AC (the current, not the climate) at 500 some volts; there is indeed boost circuitry to get from the 200 some battery volts, as well as change AC to DC and vice versa.

The gear ratio in the HSD is fixed, the output wheel speed depends on the input from the ICE and motors and the direction the smaller motor is spinning. Above 42 mph the ICE is always spinning, even during braking, to keep the electric motors within speed specs.

As for regen braking technique, or conventional braking for that matter, a gradual application of the pedal gives you more stopping control. I don't think you need to worry about taxing the electrical circuits if you press fairly hard. If you exceed the absorption capacity of the battery, you get friction braking anyway.

The Prius is computer controlled and the Toyota engineers were conservative in programming the braking and battery computers. That’s why the battery has such a long life and the car "burns" excess charge when the battery display goes solid green.

If you're in the Portland area, check out the local group.

 

PC acting up, double post.

 

The high limit from regenerative braking comes from the battery's ability to absorb charge. If you generate too much current, there is no place to put it. Regenerative braking is limited to about 20 kW, where MG2, which does the regenerative braking, is good for about 50 kW.

MG1 and MG2 are both hybrid synchronous permanent magnet motors, which means they operate as both permanent magnet motors and induction motors. It allows for more power from a smaller permanent magnet, while still maintaining good efficiency. Voltage is boosted and converted by the inverter, which is also the synchronous motor controller.

MG2 is directly connected without gearing. As you have stated, MG2's speed becomes too low for effective regeneration at around 8 mph, so at this point the Prius gives up and switches to friction brakes.

I wouldn't worry about loss of magnetization. The life of the motors will exceed the life of the car, unless they lose cooling.

Tom

 

Regeneration is under computer control to protect the system and optimize battery life while providing the braking effort being called for. There's little point in trying to second guess it; just brake as needed.

If you're really asking about maximizing fuel economy, there are lots of threads with specific advice.

 

Does battery charging efficiency fall off in the upper half/quarter/whatever as opposed to the mid range/bottom of the charge rate band? Is it more efficient when the battery is cold, cool, warm, hot...etc? I realize that little good happens at extremes.

I've wondered whether or not a slow auto regen braking was more efficient than braking manually in say half or 1/4 the distance (assuming that a stop will be necessary either way.)

 

There is a trade off between the efficiency of MG2 and the ability of the battery to absorb charge. I don't recall the exact numbers, but it seems to me that the most efficient rate is about half of maximum, or 50A. A long slow regen looses energy on the generating side, where a high rate regen looses energy heating the battery.

The general thinking is that you are better off braking a bit more aggressively than doing a long slowdown. That said, you are better off coasting than braking if you can do it.

Tom

 

Ahem, a lot less than 25A. Our NiMH batteries are rated at 6.5Ahr giving a 1C charge rate of 6.5A. But when testing batteries, it is common practice to use 0.1C.

Don't get me wrong, any regeneration is better than none but less is best. If you can ballistic glide to a stop without be a safety hazard, that is best of all.

Bob Wilson

 

The battery accepts/supplies more power when it is hot.
However, there are another stories for cell life and etc.
Plastic Case Prismatic Module | Panasonic EV Energy Co., Ltd.

Ken@Japan

 

The service manual does show diagnostic trouble codes that are reserved for indicating reduced magnetism in MG1 or MG2. I don't know whether anyone ever has or will see those codes.

-Chap

 

Take a spin in a G1 Prius for a non-subtle transition. I'd go so far as to describe the sensation as jerky.