Regenerative Braking Physics Question

[ERD50]

I guess I'm so jaded by all the 'breakthroughs' that die on the vine, I was pleasantly surprised to see that this carbon foam lead-acid battery by Firefly has actually gone into production!

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What is the stage of development is Firefly technology? The Oasis battery (Group 31 size) is commercially available in 12v and 4 v models. These batteries use carbon foam negative plates. Larger format batteries are expected to be in production by the fourth quarter of 2015. Batteries with carbon foam positive plates will become available in 2016.

So these are the single plate foam versions. But the fact that they are actually shipping these gives me some confidence in their 2016 goal for replacing both plates.

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The Firefly batteries have four times the life and two times the energy density of a traditional lead acid battery

It looks like that's still about half the energy density of Lion? I'll have to poke around to see how much better they expect the generation with two foam plates to be.

-ERD50

[MBAustin]

FWIW, my Prius gets better mileage at moderate highway speeds (55-65) than in city traffic with frequent stops. The battery is not just charged by regen braking but also by the gas engine, so the electric engine is always providing some of the power. Even though you get significant battery charge from stopping at a traffic light, the acceleration back to 30-35mph is almost completely from the gas engine at a low MPG (at which point if you back off the accelerator, you can go some distance on electric only),

[ERD50]

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Originally Posted by MBAustin FWIW, my Prius gets better mileage at moderate highway speeds (55-65) than in city traffic with frequent stops. The battery is not just charged by regen braking but also by the gas engine, so the electric engine is always providing some of the power. Even though you get significant battery charge from stopping at a traffic light, the acceleration back to 30-35mph is almost completely from the gas engine at a low MPG (at which point if you back off the accelerator, you can go some distance on electric only),

I'm not sure the bold parts are accurate.

so the electric engine is always providing some of the power. - Unless you are accelerating, or running under battery power only, the electric motor won't normally be providing any power. An exception to that would be if the regen braking had already topped off the battery, I think they will 'use up' some of that to propel the car, so that the next regen cycle can be stored rather than wasted.

There is no point in using a generator to charge the battery to run the motor in a Prius (as opposed to a serial hybrid), there is loss in the conversions. Other than acceleration boost (and the exception I gave above), it is more efficient for the gas engine to drive the wheels directly, and skip the conversions. Even the Chevy Volt, which is mostly a serial hybrid, does some of this for efficiency.

Another mode could be, the battery is very low due to little regen. But they need the electric motor boost to be ready to handle any required acceleration, they could use the gas engine to do some battery charging. The steady state of the engine won't be affected much, and then the battery will be ready to boost the acceleration for a short burst. But they wouldn't run the drive the motor at the same time (I think the motor and generator are the same piece of hardware in the Prius?).


the acceleration back to 30-35mph is almost completely from the gas engine - Acceleration is where the electric motor gives max boost. It might be true that most of the energy comes from the gas engine (especially at those speeds), but if it was 'almost completely from the gas engine', there would be little point in adding the electric motor and batteries!

Or maybe almost all the electric motor boost is below 30 mph? I don't know - I would assume it would come into play anytime you accelerated?

-ERD50

[MBAustin]

I'm not a Prius engineering expert, my comments were based primarily on my 5 years of Prius driving experience, including watching the displays that show roughly what's going on with the 3 motors (2 electric & 1 gas).

For those who really want to get into the details, there's a much better forum:

PriusChat

Two other interesting sites:

Explanation/demo of the Power Split Device (PSD) that determines which engines do what at any given time:

Toyota Prius - Power Split Device

Detailed explanations of "What's Going on As I Drive" a Prius

Graham's Toyota Prius
"The harder you press the accelerator, the more torque the ICE produces. This increases both the mechanical torque though the ring and the amount of electrical power generated by MG1 for MG2 to use to add still more torque. Depending on various factors such as the battery state of charge, the road grade and exactly how hard you press the pedal, the computer might draw extra power from the battery to boost MG2's contribution. This is how highway passing acceleration is achieved with only a 70 horsepower ICE in such a big car. On the other hand, if power demand is not that high, some of the electricity produced by MG1 may be used to charge the battery, even while accelerating! The important thing to remember is that the ICE both drives the wheels mechanically and drives MG1 forwards enabling it to generate electricity. What happens to that electricity and whether more electricity is taken from the battery depend on complex factors which may be beyond our ability to fully figure out."

[NW-Bound]

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Originally Posted by clifp Here is nice video which shows the measurement for a Tesla on regenerative breaking...

I watched the energy consumption display, and it went from 1.7 kWh at rest to 2.9 kWh at the top speed of 100+mph, then dropped down to 2.4 kWh at rest again.

So, the car expended 1.2 kWh to get to that top speed and recovered 0.5kWh or 42% of that energy. That's not bad considering the energy irretrievably lost in the moving part friction, tire rolling resistance, and aerodynamic drag.

By the way, 1.2 kWh is the energy that can lift this 4,800-lb car a height of 660 ft. And in order to charge the 85 kWh battery from empty by regen braking, you need to coast the car from a height of no less than 47,000 ft.

The above numbers give me a better perspective on the amount of energy that is required to push a car at highway speed. It's huge.

[Nords]

Quote:

Originally Posted by MBAustin Graham's Toyota Prius "The harder you press the accelerator, the more torque the ICE produces. This increases both the mechanical torque though the ring and the amount of electrical power generated by MG1 for MG2 to use to add still more torque. Depending on various factors such as the battery state of charge, the road grade and exactly how hard you press the pedal, the computer might draw extra power from the battery to boost MG2's contribution. This is how highway passing acceleration is achieved with only a 70 horsepower ICE in such a big car. On the other hand, if power demand is not that high, some of the electricity produced by MG1 may be used to charge the battery, even while accelerating! The important thing to remember is that the ICE both drives the wheels mechanically and drives MG1 forwards enabling it to generate electricity. What happens to that electricity and whether more electricity is taken from the battery depend on complex factors which may be beyond our ability to fully figure out."

Translation:
"The car has a continuously variable transmission, but the battery boost means that you can still peel rubber accelerating uphill without worrying about a clutch."

Of course you can see the battery charge indicator drain right before your eyes, but by the time it gets low you're already going 75 MPH.

Or so I've heard.

[clifp]

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Originally Posted by NW-Bound I watched the energy consumption display, and it went from 1.7 kWh at rest to 2.9 kWh at the top speed of 100+mph, then dropped down to 2.4 kWh at rest again. So, the car expended 1.2 kWh to get to that top speed and recovered 0.5kWh or 42% of that energy. That's not bad considering the energy irretrievably lost in the moving part friction, tire rolling resistance, and aerodynamic drag. By the way, 1.2 kWh is the energy that can lift this 4,800-lb car a height of 660 ft. And in order to charge the 85 kWh battery from empty by regen braking, you need to coast the car from a height of no less than 47,000 ft. The above numbers give me a better perspective on the amount of energy that is required to push a car at highway speed. It's huge.

47,000 foot that is a big mountain. My house is 1,000 feet above sea level and I spend a lot of energy moving the Tesla up the hill a once or twice a day.

In another video they pointed out that 1.2 KWH cost about $.10 in Florida where they film a lot of the drag racing videos. From a sheer entertainment value a dime to a 1/4 mile drag race in a Tesla is hell of a bargain.

Too bad it take $100K upfront investment to spend the dime.

[Nords]

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Originally Posted by clifp Too bad it take $100K upfront investment to spend the dime.

But being able to use the phrase "Ludicrous acceleration" is worth it!

[NW-Bound]

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Originally Posted by clifp 47,000 foot that is a big mountain. My house is 1,000 feet above sea level and I spend a lot of energy moving the Tesla up the hill a once or twice a day...

Tesla says that the 85kWh battery can move the Tesla 320 miles at a constant speed of 55 mph. Seeing that this same amount of energy can lift a 4,800-lb car up 47,000 ft gives an idea of how much it takes to move a car sideways at highway speeds. And 55mph is not fast.

Here's some more simple calculations for one to get some insight. Imagine a long ramp of 320 miles, sloping down such that one end is 47,000 ft higher than the other. Take a Tesla, put it in Neutral (if it has that mode), and the car will coast at a constant 55mph down the ramp.

This ramp that has the car converting potential energy to exactly cancel out the aero drag and rolling friction has a slope of 2.78% or 1.6 degrees. That shows that the Tesla has a fairly low drag. When I coast down a 6% slope in my RV, I am not stupid to put it in Neutral, but in Drive, man, the darn thing hardly accelerates from 70 mph. Its aero drag is that bad!

Back to the Tesla, that constant 55mph down that slope means the drag is 133 lbs at that speed. On level land, the power to fight that 133-lb force at the speed of 55mph is 20 hp.

The aero drag effect on cars is huge. My RV only gives me 8-8.5 mpg at low elevations near sea level, and I generally drive at 60 mph. On trips in the high Western states like Colorado and Wyoming, I had as high as 9.5-10 mpg. When one considers that drag is proportional to air density, and that air at 7,000 ft is 80% as dense as at sea level, the result is not surprising.