# Regenerative braking and batteries



## major (Apr 4, 2008)

DJSirMatthew said:


> Newbie here, so I hope you don't mind the dumb questions...
> 
> I'm looking for exact information for why the energy from regenerative braking is usually saved into supercaps and not batteries, while batteries are usually used for propulsion. I understand that with braking a lot of energy comes available at a very short time, so I suppose the load current will be high. But how high (consider a race car)? And why is it too high for batteries?
> 
> ...


Hello DJSir,

This


> the energy from regenerative braking is usually saved into supercaps and not batteries


is untrue. In almost all cases, the battery is used for regeneration. Rarely are capacitors used although they are discussed often. Such ultracapcaitors or supercaps could be of great value in systems which employ energy storage systems that are not capable of electrical recharge such as fuel cells. But batteries used in modern BEVs are fully capable of the task of storing the energy recovered during a vehicle braking event.

Once you learn the basic parameters concerning batteries, the manufacturer's spec sheets give you the information needed to determine the requirements for the application. The amount of energy from a braking event is simply the change in Kinetic Energy (speed dependent) and the change Potential Energy (change in elevation).

How high you ask? I have heard that F1 will brake at the MegaWatt level. Even there, KERS systems use batteries. Well, special batteries (read expensive). But for the normal race cars running on battery power, the main battery pack would be fully capable of regeneration with the possible exception of the first corner, and you'll always have the friction brakes available 

Regards,

major


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## Salty9 (Jul 13, 2009)

major said:


> . . . . . the main battery pack would be fully capable of regeneration with the possible exception of the first corner, and you'll always have the friction brakes available
> 
> Regards,
> 
> major


This makes me wonder what happens with regen into a full battery


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## major (Apr 4, 2008)

Salty9 said:


> This makes me wonder what happens with regen into a full battery


That is exactly why you might need to use the friction brakes on the first corner. But most all races have a formation and/or recon lap which would burn off enough battery energy to give you headroom for the first braking event.

About the only time "regen into a full battery" is a real problem is when you fully charge your BEV on the top of a hill


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## major (Apr 4, 2008)

Salty9 said:


> This makes me wonder what happens with regen into a full battery


Or you mean functionally? If the battery voltage is at the upper limit, the negative torque command fades away per the algorithm and the driver instinctively presses the brake pedal harder applying the friction brakes. Works like a champ.


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## DJSirMatthew (Feb 9, 2013)

Ok, let's take a simple example just to see if I'm right here. Let's take a motorcycle with a weight of 260kg driving 200 km/h and decelerating to make a turn to 40 km/h in 3 seconds.

The kinetic energy would then be = 260/2*(44.4)^2 = 256.8 kJ

in 3 seconds means that gives a power of 85,6 kW

Given the voltage of the battery pack is 360V that would give a current of 237A , right?

Are Li-Ion batteries capable of handling such a current for 3 seconds? Or LiPo batteries?


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## major (Apr 4, 2008)

DJSirMatthew said:


> Ok, let's take a simple example just to see if I'm right here. Let's take a motorcycle with a weight of 260kg driving 200 km/h and decelerating to make a turn to 40 km/h in 3 seconds.
> 
> The kinetic energy would then be = 260/2*(44.4)^2 = 256.8 kJ
> 
> ...


The bike you see in my avatar had such a battery. It was about 11 kWh, so around 30 Ah. Therefore 237 A would be about 8C. No problem for it. That was A123. Most LiPo would have no problem. Even PbAcid would handle it.

Say you had half the size battery or talked about a 3 to 6 second 10 or 15C blast. I don't see a problem. Most decent batteries can take short duration charge at least as high as the discharge peaks. On some EVs, the regen peaks could be higher, but that also has to be within the propulsion system capability.

And, IMO, with a motorcycle, you'd never be able to get that much braking torque on the rear due to weight transfer forward 

edit: Your energy calc is off. KE(1)-KE(2) does not = ½M*(V1-V2)²


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## Duncan (Dec 8, 2008)

Hi DJSir

Re-gen is never used as the main method of hard braking - the energy transfer is too great

The limiting step is not the power into batteries but the motor power

Most modern cars can brake pretty close to the adhesion limits of the tires

Car - 2000Kg
Tire limit - 80%
Retarding force - 80% x 2000Kg x G = 16,000N

100 mph = 160Kmhr = 44M/sec 

Power = Force x Distance/time = 44 x 16,000 = 704Kw = 940Hp

There are very few four wheel drive cars with nearly 1000Hp of available power

Note the power required is proportional to speed so a 200mph car would need 2000Hp of available power

Good friction brakes can operate absorbing that sort of power level for the few seconds that are required


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## subcooledheatpump (Mar 5, 2012)

Also consider that some inverter drives have a brake chopper that can activate a dynamic brake resistor in cases when the battery is full. They may not be as common in EVs but in other equipment with motors that need to be quickly stopped, the resistor can dissipate what other systems (capacitors, batteries, or friction brakes) cannot absorb


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## Electroid (Nov 23, 2012)

I have a Solectria Force, the way my car works is the power goes back into the traction batteries, 
according to the gauge I have it will generate around 70 amps of charging power, which will slow the car down about like pushing around 50% of the brake pedal, this will stop the car without using the brakes about 90% of the time in my normal (somewhat slower) driving.

it does not matter how fast I am going, it does not appear to go much over 70 amps of charging power, it just takes a long distance to stop, and if I am trying to stop going down hill, it takes even a longer distance to stop. my point is that there is a limit to how much braking power you have and it can't go over that limit.

when I first drive the car after charging, for about the first mile, when I use the regen it will limit it to something around 30 amps, if it goes over that I get a type of grinding noise and no more amps on the meter, so it is set up to prevent over charging a full battery by not allowing 70 amps of charging until the battery has been used for about a mile.


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## onegreenev (May 18, 2012)

I blew a controller using a regen setting that was too high. Bummer. Now its an expensive door stop that stinks of magic smoke.


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## kennybobby (Aug 10, 2012)

*Blind leading the blind--what brake pads can do that?*



Duncan said:


> Most modern cars can brake pretty close to the adhesion limits of the tires
> Car - 2000Kg
> Tire limit - 80%
> Retarding force - 80% x 2000Kg x G = 16,000N
> ...


Formula 1 carbon?

Your power number is way off--to decelerate from 100mph in one second is about 4.5g, not the 0.8g stop that you were trying to calculate, which would take about 5.7 sec.


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## marc02228 (Jan 15, 2011)

major said:


> How high you ask? I have heard that F1 will brake at the MegaWatt level. Even there, KERS systems use batteries. Well, special batteries (read expensive).


Where did you read that?
I heard from a quite reliable source, that they use capacitors. Some kind of prismatic cap's.


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## major (Apr 4, 2008)

marc02228 said:


> Where did you read that?
> I heard from a quite reliable source, that they use capacitors. Some kind of prismatic cap's.


http://www.mavizen.com/products/a123-ahr18700-f1-kers-cells/ 

This is one place recently which infers batteries are used. I have read a number of articles over the years talking of batteries in the KERS. But I am afraid I don't bookmark all those, so I guess it is mostly hearsay on my part.

I have worked a lot with both batteries and ultracapacitors. Some of the variety of product mixes and crosses over the line between capacitor and battery IMO. Called the asymmetrical capacitor cell, it has one plate from a cap and the other from a battery. Which is it? Cap or Bat


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## marc02228 (Jan 15, 2011)

major said:


> http://www.mavizen.com/products/a123-ahr18700-f1-kers-cells/
> 
> This is one place recently which infers batteries are used. I have read a number of articles over the years talking of batteries in the KERS. But I am afraid I don't bookmark all those, so I guess it is mostly hearsay on my part.
> 
> I have worked a lot with both batteries and ultracapacitors. Some of the variety of product mixes and crosses over the line between capacitor and battery IMO. Called the asymmetrical capacitor cell, it has one plate from a cap and the other from a battery. Which is it? Cap or Bat


Hmm, interesting. Maybe they have different solutions to store the energy.


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## TEV (Nov 25, 2011)

Electroid said:


> I have a Solectria Force, the way my car works is the power goes back into the traction batteries,
> according to the gauge I have it will generate around 70 amps of charging power, which will slow the car down about like pushing around 50% of the brake pedal, this will stop the car without using the brakes about 90% of the time in my normal (somewhat slower) driving.
> 
> it does not matter how fast I am going, it does not appear to go much over 70 amps of charging power, it just takes a long distance to stop, and if I am trying to stop going down hill, it takes even a longer distance to stop. my point is that there is a limit to how much braking power you have and it can't go over that limit.
> ...


All those numbers are coming from custom settings in your controller, the maximum regeneration Amps should be set according with controller capabilities an battery pack specifications.


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## Duncan (Dec 8, 2008)

*Re: Blind leading the blind--what brake pads can do that?*



kennybobby said:


> Formula 1 carbon?
> 
> Your power number is way off--to decelerate from 100mph in one second is about 4.5g, not the 0.8g stop that you were trying to calculate, which would take about 5.7 sec.


Hi kenny

My example was for a road car - F1 cars do decelerate at over 4G but this is partial by aerodynamic drag and partly by down force increasing the load on the tires - not to mention sticky racing cars

Some road cars may be able to to decelerate at 1G - but not at 100mph

F1 example
Car - 650Kg

Retarding force - 300% x 650Kg x G = 19,500N
(assuming 3 G from tires)

100 mph = 160Kmhr = 44M/sec 

Power = Force x Distance/time = 44 x 19,500 = 858Kw = 1144Hp

at 200 mph

88 x 19,500 = 1716Kw = 2288Hp


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## Duncan (Dec 8, 2008)

Hi Major

F1 cars
The braking is in the megawatt range - but that is done by the fancy carbon discs

KERS gives about an 80Hp boost - so I would expect it to be re-charging at a similar level


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## major (Apr 4, 2008)

Duncan said:


> Hi Major
> 
> F1 cars
> The braking is in the megawatt range - but that is done by the fancy carbon discs
> ...


Yes Dunc,

I never said anything to the contrary  

major


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## Jesse67 (May 12, 2009)

Well I've got my thundersky pack limited to 300A on regen which is 3C, the max charge current on the spec sheet, they can probably handle more for short periods of time. However, this gives quite an aggressive stop and it can lock up or skid the back tires if there is any gravel, snow, or water on the road. A fwd car could recover at higher power levels due to better traction as a result of weight transfer forward. But really, if I need to stop faster than that I have more to worry about than the energy I'm loosing! 

The amount of energy you could recover will be your initial kinetic energy x tire efficiency x drive line efficiency x motor efficiency x controller efficiency x battery efficiency minus the energy normally lost over the time and distance it takes you to stop due to rolling resistance and aerodynamic drag. 

I've got no idea what tire efficiency might be but some energy is definitely lost due to a certain % slip (not locked up) under high load (assuming an aggressive stop). Battery efficiency depends on current and temperature, I've measured between 86% and 94% with my cells. So as an example, for my truck with the AC50, say 95% (tires) x 95% (driveline) x 90% (motor) x 90% (controller) x 90% (batteries) = 65.8%. I won't make a guess about rolling resistance and aero losses during the stop but a coast down test from the same speed could tell you something about that with a bit of math. So at best you'll have less than 65% of the initial kinetic energy back in the battery at the end of the stop, and all those efficiencies and losses also come into play as you take off again! Interesting to think about though, I may have to try a controlled stop with just regen from a set speed and I'll see how many Ah I get back and at what approximate voltage. This is what would be going into my batteries. Multiply this by the battery efficiency, calculate the kinetic energy I had at that speed and I could get the actual fraction of energy recovered.

Have Fun!

Jesse


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## Electroid (Nov 23, 2012)

I did a very unscientific test with my car and I drove up a hill and measured how many amp hours it took, then I turned around and went the same distance back down the hill and I ended up getting around 50% of the amp hours back from regen.


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## TEV (Nov 25, 2011)

Electroid said:


> I did a very unscientific test with my car and I drove up a hill and measured how many amp hours it took, then I turned around and went the same distance back down the hill and I ended up getting around 50% of the amp hours back from regen.


Why is unscientific ? Unless you counted the Amps by touching the wires .

Very good job.

P.S. Also some information about the hill are required, otherwise the 50% number is meaningless.


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## DJSirMatthew (Feb 9, 2013)

So, summarizing, on motorcycles it is only "engine braking" that is converted to electric energy and not the "mechanical braking". Is that because of too much power for the engine to handle? Or are there disadvantages to it that undo the advantages (too much weight, unstability because of shift of the center of gravity,...). I know I've read an article somewhere of Chip Yates with a KERS system that recovered energy from mechanical braking...

And maestro, did you battery pack exists of all the same cells? Or did you have different cells for propulsion and other for regeneration?


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## major (Apr 4, 2008)

DJSirMatthew said:


> So, summarizing, on motorcycles it is only "engine braking" that is converted to electric energy and not the "mechanical braking". Is that because of too much power for the engine to handle? Or are there disadvantages to it that undo the advantages (too much weight, unstability because of shift of the center of gravity,...). I know I've read an article somewhere of Chip Yates with a KERS system that recovered energy from mechanical braking...
> 
> And maestro, did you battery pack exists of all the same cells? Or did you have different cells for propulsion and other for regeneration?


Hi DJSir,

I don't know how you arrive at your summation, but with the motorcycle you are able to effectively brake regeneratively at levels higher than ICE engine braking. There is the limit of traction on the rear wheel during braking which limits the amount of braking torque applied to that wheel regardless of where that braking torque originates (motor or friction).

Chip Yates tried to solve this problem by incorporating a mechanism to couple the front wheel on the motorcycle to the electric motor drive so that the regen torque could be applied to that front wheel where the weight is transferred during braking. While the concept is valid, the implementation was unduly complicated and robbed the vehicle of so much space (and mass penalty) that there was insufficient room for a proper sized battery. He dropped it.

The e-motorcycles I work on all have uniform battery packs. I have, years ago, worked with a 4-wheeler using combination battery concepts including ultracap/PbAcid and NiCad/PbAcid. We did run these systems in the lab and in the parking lot but none ever made it to the race track. I wish we'd had Lithium back then 

Regards,

major


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## DJSirMatthew (Feb 9, 2013)

You're right major (sorry I got your name mixed up).

You're saying that the brake torque on the rear wheel is limited. Someone told me that the energy during braking with a motorcycle mainly comes from the front wheel (factor 4-to-1 he mentioned). Is that true? How come?

Thank you for all the helpful information!

Regards


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## major (Apr 4, 2008)

DJSirMatthew said:


> You're saying that the brake torque on the rear wheel is limited. Someone told me that the energy during braking with a motorcycle mainly comes from the front wheel (factor 4-to-1 he mentioned). Is that true? How come?


They call it "weight transfer". That might not be the technically correct description of what happens, it does infer the result. The same thing happens in a 4-wheeler but there is a lot more weight on the rear to begin with so it is less noticeable. I guess you could say the inertia of the bike and rider tend to keep traveling forward as the reverse torque on the wheels tend to retard forward motion, so the rider is being forced over the handlebars  This tends to stand the bike up on the front tire (stoppie). And reduce the downward force on the rear tire (reducing traction). This is why you see large double discs on the front wheel and a single small disc on the rear.


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## Jesse67 (May 12, 2009)

How much weight transfer you have depends on the height of the center of gravity relative to the length of the wheel base. Something like a F1 car will have very little transfer but a sport bike will have a lot due to the short wheel base and relatively high C of G. That's whey they can do a stoppie! Interestingly a mini truck with an empty bed can also do one apparently....

https://www.youtube.com/watch?v=k2D6G1zkceA

The height of the C of G above the road acts like a lever tilting the vehicle forwards while the braking force pushes back on the tire contact patches. Try braking on a pedal bike with the front and back brakes independently and you'll gain some first hand experience..... wear a helmet!


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