Confused by the talk about not using the electric motor

Gliding all the way to a stop is the most efficient, but not always practical. It works like this:

1) Anytime you brake you are wasting energy.
2) Regen braking is less wasteful than friction or engine braking.
3) Steady regen braking over a longer period is more efficient than hard regen braking for a short period.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Sep 18 2007, 03:15 PM) [snapback]514463[/snapback]</div>

In general I agree with your 3 statements. Where it can get confusing is when you combine them.

Gliding is very efficient, max regen is less efficient than a more gentle regen.
But if you combine gliding with max regen, how does it compare with gentle regen the whole distance? Does the efficiency of the glide offset the inefficiencies of the hard regen?

I suspect that it depends on how much sooner you start the glide than the regen. If the gentle regen and the glide are both started at the same distance from the planned stop, I "think" the gentle regen wins out in terms of efficiency. If gliding means you will begin your stopping process from a greater distance from the planned stopping point, then I "think" it may be possible to be more efficient with the long glide depending on how much greater the distance is.

 

<div class='quotetop'>QUOTE(qbee42 @ Sep 18 2007, 03:15 PM) [snapback]514463[/snapback]</div>

This makes the most sense, but I'll reword it a different way...... recapturing energy is always a dicy challenge.

"Time" is always a factor in the equation.
I'm not an authority on the subject, but generally speaking there are always curves where efficiency starts going out the window.
Anytime you are trying to capture allot of energy in a small amount of time, the efficiency will always be less than captureing that same amount of kinetic energy over a longer time.

Some of that just purely has to do with cost of parts verses whats practicle.
Its awesome that braking allows some regen, but I really suspect the most "efficient" is when its done over more time or a longer distance, hence coasting or gently braking.

Its not realistic to think we can romp on the brakes and save as much kinetic energy as gently coasting.
True, the 50K generators make more power for a given instant, but when the dust all settles, there will be less saved than with gentle coasting or braking.

This is of course just my hunch and my opinion based on no real evidence or facts..... some times common sense is all it takes to figure things out.

However as Mark Twain said, the only problem with common sense is it isn't common!

 

<div class='quotetop'>QUOTE(windstrings @ Sep 18 2007, 08:43 PM) [snapback]514596[/snapback]</div>

The battery has an internal resistance and the current has to flow through it. This generates waste heat using the following formula:

(I**2)R

I - current, Amps
R - resistance

So a 1 ohm battery resistance (chosen to make the math easy) with say 10 A. of coasting regeneration would make:

(10*10)*1 = 100 watts of wasted heat

Now push on the brake pedal enough to maximize regeneration current and suddenly:

(100*100)*1 = 10,000 watts of wasted heat (OWCH!!!)

Meanwhile, down at the cell molecular level, the voltages might climb enough that instead of forcing H(2) into the metal matrix, it decides to do some other, irreversible reactions . . . say generate some H(2) gas or a little steam . . . this is bad.

I typically see coast down regen current in my 03 Prius of ~15-20A but braking while descending a steep hill can exceed 100 A. At the bottom of the hill, I've seen bulk battery temperature rise a couple of degrees C. I've got some data if anyone wants to see more detail (and suddenly a hundred fingers recoil from the keyboard . . . <grins>.)

Bob Wilson

 

<div class='quotetop'>QUOTE(vincent1449p @ Jun 17 2005, 11:43 PM) [snapback]99585[/snapback]</div>

Thanks for your common sense driving. I agree. Why maim or kill yourself trying to get the highest MPG to post on this forum

 

<div class='quotetop'>QUOTE(bwilson4web @ Sep 20 2007, 10:31 AM) [snapback]515078[/snapback]</div>

I understand the value of 1 Ω is chosen to make the maths easy but 10 kW is a bit exaggerating.

That's why battery resistance for autos are typically in the mΩ range. The Prius HV battery is around 15 mΩ according to the graph here.

(100*100)*0.015 = 150 W

 

<div class='quotetop'>QUOTE(vincent1449p @ Sep 23 2007, 10:33 PM) [snapback]516734[/snapback]</div>

And yet the math works out the same:

(10*10)*0.015 = 1.5W

Either way you are looking at an increase in heat generated of 9900%

The thing that is ignored in either set of calculations is the fact that if the brakes are applied at the same starting speed, the 10A braking will require a greater amount of time to stop.

If the 100A braking can stop the car in 10sec, and the 10A braking requires 1000 sec, then there really isn't any benefit to the gentler braking.

150W * 10 sec = 1500 W*sec
1.5W * 1000 sec = 1500 W*sec

I don't have the time to figure the math right now, but I'm pretty sure that the stopping time does not increase nearly as quickly as the Watts generated decreases, but the point is that you can't just compare Amps times Ohms, you need to take stopping time into consideration as well if you are trying to determine how much extra heat is generated using one stopping force versus another.

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Sep 25 2007, 09:42 AM) [snapback]517388[/snapback]</div>

The mass and surface area of the battery remains constant as do the thermal characteristics. In the shorter interval, there is less time for the excess heat to radiate to the surface and away. So the inside of the battery gets warm, much warmer than it does over the longer, slower rate. Overtime, the battery gets warmer and warmer until the battery fan comes on and attempts to cool the battery. If not checked by the fan, the internal battery temperature can continue to increase until the electrolyte boils off and irreversible changes destroy the battery.

Last summer, I did a series of severe hill climb tests over a 1,000 ft. rise. The climb up resulted in significant battery currents drawing energy out, an (I**2)R heating followed by descents with (I**2)R heating going down. It was 100F at the bottom and 95F at the top and I was surprised on a break at the top when the battery fan came on after I parked. I let the car cool down before returning home.

Thirty years ago, the voltage regulator on the generator failed and it pegged the charging amp scale. I quickly landed at an airport and when I looked at the battery, it was venting steam. Again, I let it cool, used the battery to start the engine and flew all the way back with the master switch off.

Bob Wilson

 

I tried this today
I found it virtually impossible to hold a dead band especially at speed

There seem to be two main modes
ICE driving wheels + supply to motor which also drives wheels and charges battery
ICE driving wheels supplemented by power from battery through motor to wheels

I can hold it so it's flicking between one and the other
Trouble is I found that after about 50 miles doing this my mpg was 5mpg down
My road concentration wasnt too good either!

What am I missing?