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Regenerative Braking Physics Question
Old 07-10-2015, 12:11 PM   #1
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Regenerative Braking Physics Question

During a test drive of a Nissan Leaf, the salesman said, "Of course you get better range for city driving, due to the regenerative braking."

That seemed to make sense until I thought about it later.

Assuming you get back less energy when braking than you use when accelerating, his statement can't be right, can it?

Compare:

1. Accelerate 0 to 30, drive 50 miles, decelerate 30 to 0.

2. Drive 50 miles in the city, accelerating to 30 and decelerating to 0 multiple times.

I'd expect situation 1 to use less energy.
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Old 07-10-2015, 12:22 PM   #2
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Perhaps what he was trying to say was: "You will get more of an advantage of an electric vehicle over a conventional vehicle with city driving".

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Old 07-10-2015, 12:39 PM   #3
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They can actually get better milage in the city than on the highway. The regen braking would be a contributor, but I think higher highway speeds and wind drag are major factors in lowering highway milage.
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Old 07-10-2015, 12:59 PM   #4
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... but I think higher highway speeds and wind drag are major factors in lowering highway milage.
This. Regen will never recapture the energy 100%. You never get back all the energy you put in to accelerate. So, bottom line, regen can't gain energy. The faster you go the more the wind load increases. I've read that load can be high enough on some cars that running the AC is more efficient than opening the windows!
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Old 07-10-2015, 02:43 PM   #5
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Drag increases with the square of speed. So 0 to 30 to 0 with a good brake regen may use less total energy than flying along at 60 with no braking. I think he could be right, but it's not as simple as he's making it sound.
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Old 07-10-2015, 02:57 PM   #6
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Quote:
Originally Posted by TromboneAl View Post
During a test drive of a Nissan Leaf, the salesman said, "Of course you get better range for city driving, due to the regenerative braking."

That seemed to make sense until I thought about it later.

Assuming you get back less energy when braking than you use when accelerating, his statement can't be right, can it?

Compare:

1. Accelerate 0 to 30, drive 50 miles, decelerate 30 to 0.

2. Drive 50 miles in the city, accelerating to 30 and decelerating to 0 multiple times.

I'd expect situation 1 to use less energy.
Yes situation 1 uses less energy, but the salesman was saying you would get better range because without regen, the brake energy is converted to heat and wasted to the atmosphere. With regen, some of the energy can be stored and used for propulsion. City driving (e.g. high stop density) increases regen utlilization compared to highway driving.
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Old 07-10-2015, 03:12 PM   #7
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My understanding is that hybrids often get their best mileage in town. I'm assuming it is the combo of region. PLUS the lower speeds leading to lower drag. I'm guessing there are forums on the Leaf and/or all electrics. I know there are on hybrids. You might see what they have to say. Enjoy the process.
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Old 07-10-2015, 03:27 PM   #8
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Yes situation 1 uses less energy, but the salesman was saying you would get better range because without regen, the brake energy is converted to heat and wasted to the atmosphere. With regen, some of the energy can be stored and used for propulsion. City driving (e.g. high stop density) increases regen utlilization compared to highway driving.
^^^ He means this.^^^^^^^ You get better mileage given the same number of stops/slow downs that would require conventional braking.
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Old 07-10-2015, 03:31 PM   #9
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+1 on what others said.

To paraphrase, lower speed travel should result in better fuel mileage due to lower aerodynamic drag, whether the engine is internal combustion or electric. However, city driving is also stop-and-go, and that hurts the fuel performance of the ICE car relative to the electric car because the latter recovers some of that energy with regenerative braking. In addition, while stopping, the ICE still burns fuel while idling, while the electric motor is shut off.

From what I have seen, ICE cars have lower city MPG than highway MPG, but electric or even hybrid cars can have better range in city than on highway.
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Old 07-10-2015, 05:12 PM   #10
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Other posters have covered it, but let me express it just a bit differently...

Quote:
Originally Posted by TromboneAl View Post
...

Compare:

1. Accelerate 0 to 30, drive 50 miles, decelerate 30 to 0.

2. Drive 50 miles in the city, accelerating to 30 and decelerating to 0 multiple times.

I'd expect situation 1 to use less energy.
Yes, but if you are comparing to highway driving the comparison should be:
1 (highway). Accelerate 0 to 55 mph, drive 50 miles, decelerate from 55 to 0.

2 (city). Drive 50 miles in the city, accelerating to only 30 mph and decelerating to 0 multiple times.
So as others have pointed out, the conventional ICE car and the EV face pretty much the same situation on the highway.

In the city, the stop and go requires a lot of energy from the ICE to accelerate, and to idle, and the energy to stop is wasted. For the EV, less idle wasted (unless the heater or AC is on?), and recovered energy when stopping.

If the EV gets better city range than highway range, that must mean the wind drag is a bigger effect than the acceleration/regen/idle.

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Old 07-10-2015, 05:18 PM   #11
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Originally Posted by jazz4cash View Post
Yes situation 1 uses less energy, but the salesman was saying you would get better range because without regen, the brake energy is converted to heat and wasted to the atmosphere. With regen, some of the energy can be stored and used for propulsion. City driving (e.g. high stop density) increases regen utlilization compared to highway driving.
But that doesn't fully answer his question.

Quote:
Assuming you get back less energy when braking than you use when accelerating, his statement can't be right, can it?
Yes, the regen helps, but as T-Al mentioned, it isn't even 100% efficient, so stop/go alone can't increase the range. That would take greater than 100% efficiency (perpetual motion machine talk).

You need to add in wind drag and compare stop/go at city speed to constant highway speed to see why city range can be greater than highway range.

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Old 07-10-2015, 05:29 PM   #12
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Very good, ERD50. I, along with others I believe, missed T-Al talking about driving 30 mph on the highway.

What T-Al was thinking is that between the constant 30-mph speed and case 2 of repeatedly accelerating to 30mph then slowing down (thus having an average speed lower than 30mph), the constant speed case will be more energy efficient, even with regenerative braking for the varying speed case. This is likely true.

We missed his leisurely 30-mph drive on the highway because while he might be able to do it where he lives, it would incite road rage in most locations.
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Old 07-10-2015, 08:40 PM   #13
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I used battery equipment for over 20 years in an underground environment on track haulage, with both hydraulic and dynamic braking. Track haulage is considered close to frictionless; but I still had to deal with elevation changes. Trust me, traveling in and out same track for up to 2 miles each way, was not easy on batteries.

I chuckle when I hear about battery use/life on some present and future transportation. If any industry needs and requires better, stronger, lighter, faster to charge, cheaper, efficient batteries, it is the underground mining industry. A LOT of money is and has been spent on battery technology since the 80's, and has little to show for it.
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Old 07-10-2015, 09:17 PM   #14
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Yes, please tell us more.

I have read that this new Lithium Iron Phosphate (LiFePO4) type is wonderful and outperforms other batteries in the long run, even though the initial cost is higher. Have you seen it tried in your application?

Been thinking about putting that into my RV, but I am not a full-time RV'er hence cannot justify the cost of $1500 to replace a couple of golf-cart batteries.
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Old 07-10-2015, 09:31 PM   #15
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The high mileage of a hybrid was explained to me as follows by a former colleague. He was describing the Prius, but I believe that the explanation applies to electric vehicles as well.

He said that the principal reason the Prius got better mileage than other vehicles, at any speed, was the gas engine was kept running at a substantially constant speed, and was supplemented at times by the battery. This is in contrast to gasoline engines which operate at a huge range of engine speeds many of which suffer low energy efficiency.

As others have said the reason any particular hybrid vehicle runs more efficiently at lower speeds arises from the energy loss to wind drag at higher speeds.

As to regen, braking my former colleague was deeply skeptical. He indicated that braking causes energy to dissipate at a much faster rate than the rate at which batteries output energy during acceleration. As a result, the batteries and electrical circuits simply can't absorb the electrical current generated from the mechanical braking energy. As a result, much of the braking energy is dissipated as heat, even in the presence of a regenerative braking system.

Returning to the original question: in all likelihood, Priuses and electrical cars get better mileage at city speeds than at highway speeds. However, this result is likely due to avoidance of wind drag and avoidance of the gasoline engine energy efficiency variability at a range of engine speeds, and not mostly due to regen braking.
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Old 07-10-2015, 09:35 PM   #16
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Here is nice video which shows the measurement for a Tesla on regenerative breaking.
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Old 07-10-2015, 11:03 PM   #17
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We did not use lithium batteries, whether it was prohibited by fed or state for underground use yet, I do not know. I am unaware of any equipment manufacturer even using them; even on the surface. We use a lot of battery equipment, diesel is prohibited in most cases, and equipment with cables are a HUGE pita. There is a great need, and I'm sure if anything was practical it would be in use. An extreme amount of downtime changing batteries, and an extreme amount of power/time requirements to recharge even at 480V 3 phase power.
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Old 07-11-2015, 01:12 AM   #18
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Many industrial equipment pieces still run on lead-acid batteries I believe. Lead-acid batteries are well-understood and safe. LiFePO4 batteries are claimed to be a lot safer than earlier types of Li battery. I have seen some test batteries intended for aviation that can be punctured by a bullet and will not turn into a fireball like earlier types. Their price is still high, and may never get down to that of the lead-acid type.
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Old 07-11-2015, 09:34 AM   #19
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Quote:
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I used battery equipment for over 20 years in an underground environment on track haulage, ...

.... If any industry needs and requires better, stronger, lighter, faster to charge, cheaper, efficient batteries, it is the underground mining industry. ...
A bit of a sidetrack here, but I recently saw a Shark Tank episode where Pat Boone and another guy were trying to pitch a car that 'runs on air'. Of course it is only propelled by (compressed) air, it 'runs' on the energy that was used to compress the air, which is done by an electric motor at a 'charging station' and, since they were marketing these in Hawaii, most of that electricity is being generated by burning oil, and since there are inefficiencies all along the way , the 'air car' is very likely burning more oil than the average mid-size car (and the air car was a tiny clown-car style)!

But of course, there were many supporting comments on the web, many of them touting the brilliance of this 'new invention' (that the oil companies will buy up and bury in the desert or some other tin-foil talk!)!

Turns out their pitch was just licensing the MDI 'product', which was supposed to come to market just about every year since 2000. You can google MDI and air car for more.

But to your comment, reading more on the history of compressed air, compressed air vehicles have been in use in mining operations since the mid-late 1800's. So much for this 'new invention'! For mines, I guess the compressed air provided the advantage of no harmful exhaust, and no combustion. But it is inefficient (largely because you lose much of the heat during compression - that is wasted), so won't replace alternatives except in these niche applications.

But a lot of non-technical people sure were impressed! It runs on air! Crush the big, bad oil companies!


Quote:
A LOT of money is and has been spent on battery technology since the 80's, and has little to show for it.
Seems to me that battery progress has been steady, but mainly in the lithium area, and it sounds like lithium was not used in mining, due to cost or safety?

So maybe what you are saying is that lead-acid batteries have not improved much? I guess I'd agree. A company in Peoria (former Caterpillar employees), were developing a lead-acid with some kind of carbon foam plates to replace the heavy lead plates. Was said to improve battery characteristics, and reduce weight. But of course there were some obstacles to overcome to get into production. IIRC, their goal was to replace both anode and cathode, I think one was easier (but far less total benefit), but I think they got some military contracts based on that first step.

Here they are: Firefly -

History Of Innovation | Firefly International Energy

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Old 07-11-2015, 10:09 AM   #20
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In the auto industry, efforts have been made to decrease mass and therefore weight, in the powertrain and in the vehicle. (ie. aluminum engine and engine pieces, plastic and carbon fiber) These allow less power to be used in acceleration and braking, making autos glorified golf carts.

No, not much advancement in lead-acid batteries, other than smart charging systems. Lead-acid batteries also provide mass and weight to increase drawbar pull, traction and counterweight requirements, but again, if a modern battery could provide longer life, faster recharge times, and be safe, hell, concrete could substitute.
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