Capacitors and regenerative braking

[RedBaron]

So I'm interested in adding a bank of capacitors to help reduce the initial load on my batteries when accelerating from a stop but my current set up uses regenerative braking. My electrinics are this order, Motors, VESC, BMS, 10s5p Li-ion pack. If I add capacitors between the bms and vesc to supply the initial burst of energy needed for take off would regenerative breaking cause an over charging issue if let's say braking down a long hill? Or would the extra power flow back to the battery once the caps are full? Just trying to understand a little more, thanks for any help.

 

[phinds]

You already HAVE regenerative braking. Why do you think adding additional storage will matter? Do you think your current storage is insufficient?

 

[berkeman]

So I'm interested in adding a bank of capacitors to help reduce the initial load on my batteries when accelerating from a stop but my current set up uses regenerative braking. My electrinics are this order, Motors, VESC, BMS, 10s5p Li-ion pack. If I add capacitors between the bms and vesc to supply the initial burst of energy needed for take off would regenerative breaking cause an over charging issue if let's say braking down a long hill? Or would the extra power flow back to the battery once the caps are full? Just trying to understand a little more, thanks for any help.

Welcome to the PF.

Can you post a schematic of your system? It sounds like it should be okay, but there are some potential issues that it would be good to check. Also post links to the datasheets for all of the components please (batteries, caps, motors, etc.). Thanks.

 

[RedBaron]

The motors are 2x 170kv 65amp each, the Vesc (motor controller) is a 200amp 10s. The bms is a 120amp continuous discharge, and the battery is an 18ah and capable of 150amp continuous discharge. I don't need or want my motors maxed out so I chose 50amps each bringing my total power required to 100amps continuous at any given time. I chose components that gave me head room for thermal efficiency and redundancy. Have I reached maximum efficiency because my battery can push 150a and I'm only asking for 100a? Would adding caps help reduce strain on the battery? I've ran out of room to add any more batteries so I was wondering if a few good caps could help.

 

[RedBaron]

Also I don't have a good way of providing a schematic. But I'll work on it.

 

[berkeman]

If you can find a PDF or JPEG file, use the UPLOAD button in the lower right of the Edit window to attach a file. Otherwise, you can post a link to the documents if you have a URL.

 

[davenn]

2x 170kv

where are you getting 170,000 V from ?

 

[Borek]

where are you getting 170,000 V from ?

kV in this context means that the motor gives 1000 rpm per every volt supplied (170 kV is a beast).

It is about as obvious as charging 2.2 Ah battery with 2C current (which means 4.4 A - C stands for battery capacity without the "h" part).

Ah, the joys of mixing meanings and symbols from different trades/hobbies

 

[RedBaron]

Lol, sorry for not being very specific. I'm super new to this so I'm doing my best to use the proper terms. Borek is correct. I'm almost done with a digital version of the schematic for you guys. Thanks for helping. This is my first project and I just felt compelled to make my own personal Mobility device. It's so exciting that the tech has gotten small enough to do this stuff.

 

[davenn]

kV in this context means that the motor gives 1000 rpm per every volt supplied (170 kV is a beast).

thanks mate ... a totally new way of describing kV for me

edit ... so my next Q is ...
is this even a valid/practical value for what the OP wants to achieve ?

 

[NTL2009]

thanks mate ... a totally new way of describing kV for me

edit ... so my next Q is ...
is this even a valid/practical value for what the OP wants to achieve ?

Actually, kv is RPM/V, not 1000 RPM/V. So 170 RPM/V in this case.

An example : https://www.ebay.com/i/153174282413...1%26rvr_ts%3D011a4cc91670ab618bd02cfdffec04b4

 

[Borek]

Oops, my bad.

 

[sophiecentaur]

Also I don't have a good way of providing a schematic. But I'll work on it.

A sketch with a sharp pencil and a photo is usually possible. Not the best but it can carry the message.

 

[sophiecentaur]

I would imagine that a super capacitor would dump current better than any battery (one of their claimed advantages). The Power that is dissipated by brakes can be much higher than what a motor / engine can supply and a capacitor could probably cope better than a battery (Edit - assuming the motor can!). However, to implement this would require some clever design because of the Voltage/Charge relationship in a Capacitor. There would be little point in just connecting battery and capacitor in parallel.

 

[essenmein]

So I'm interested in adding a bank of capacitors to help reduce the initial load on my batteries when accelerating from a stop but my current set up uses regenerative braking. My electrinics are this order, Motors, VESC, BMS, 10s5p Li-ion pack. If I add capacitors between the bms and vesc to supply the initial burst of energy needed for take off would regenerative breaking cause an over charging issue if let's say braking down a long hill? Or would the extra power flow back to the battery once the caps are full? Just trying to understand a little more, thanks for any help.

Both the batteries and caps would be in parallel right? Battery voltages do not change much between states of charge, for Li ion off the top of my head full discharge is ~2.8V/cell, max charge is 4.2V/cell but since you are probably not trying to kill your batteries you probably won't go discharging into the 0-20% SOC range, so basically your cell voltage changes by about 0.8-1.2V from "full" to empty. So then you would take your max cell change in voltage, multiply by number of cells, and you'll get the range of voltage operation for your caps. You have 10 cells in series, so your bat volts will vary from 30V to 42V. Since the batteries determine this min and max voltage to avoid damage, the energy available to use in your caps will be 1/2C(Vfull^2-Vempty^2), when you then go an math out the number of joules your batteries are delivering over that voltage and and you then work out if you want say 1/10th to come from the caps, the size of the capacitors needed will make you go and buy more batteries because you might have a shot at fitting those.

Even "super caps" are cute compared to the energy storage of electrochemical cells, and that is adorable when compared to the chemical potential energy of petrol.

Basically capacitors right next to your inverter will help reduce AC ripple current on the batteries (ie from the PWM) but it won't give you any significant energy storage for driving.

A 10Ahr 36V nominal battery stores ~1.3MJ, to have a capacitor that stores one tenth that (130kJ) over the same voltage range is about 300F, note the lack of micro in front of that F!

Oh and re power flowing to caps then battery, that's not how it works, power will flow into both at the same time, realistically the batteries will store a lot more energy than the caps as the voltage increases.

 

[sophiecentaur]

Both the batteries and caps would be in parallel right?

Why? Their charge / voltage characteristics couldn't be more different. How would the Capacitor's stored energy be significant if it is tied to the batteries? I can't think what the optimum way of connecting the two would be but in parallel would seems to be bad value - considering the cost of the capacitor bank.

 

[essenmein]

Why? Their charge / voltage characteristics couldn't be more different. How would the Capacitor's stored energy be significant if it is tied to the batteries? I can't think what the optimum way of connecting the two would be but in parallel would seems to be bad value - considering the cost of the capacitor bank.

Well I can' think of any other way you would run both batteries and capacitor energy storage together to feed the same DC bus for a motor drive. I mean you could link the caps with bi directional DC-DC to allow you to use the full voltage range of the cap, but that's likely more complicated than its worth.

I was just answering the OP question by showing how huge the cap bank would have to be even be noticeable, hence demonstrating that it doesn't make sense!

 

[sophiecentaur]

Well I can' think of any other way you would run both batteries and capacitor energy storage together

I think that, to justify the cost of a big bank of expensive capacitors, they would need to be used by an intelligent circuit. I would imagine a switch mode 'charge router' to send the charge to the Capacitor when the motor volts go higher than needed for battery charging and to use that charge first, to run the motor. Endless fun, trying to get it right, of course but you really have to do something to make the two storage devices compatible. I could be wrong but I think it would be very lucky indeed to find that a parallel connection was worth the money.

 

[essenmein]

I think that, to justify the cost of a big bank of expensive capacitors, they would need to be used by an intelligent circuit. I would imagine a switch mode 'charge router' to send the charge to the Capacitor when the motor volts go higher than needed for battery charging and to use that charge first, to run the motor. Endless fun, trying to get it right, of course but you really have to do something to make the two storage devices compatible. I could be wrong but I think it would be very lucky indeed to find that a parallel connection was worth the money.

From what I've seen mixing storage is not done. You obviously have capacitors on a system with batteries (inside the inverter or DC-DC to supply the HF AC current needed to PWM) but for bulk storage I've only ever seen super caps or batteries. Super caps actually more in heavy duty applications where the vehicle has to stop and go a lot (eg garbage trucks, delivery trucks, busses) they get filled on a single deceleration, then provide the ooomf to get the vehicle back to 20-30 before the diesels take over again but no actual real storage. For hybrids they just can't match the storage density of electro chemical and honestly modern Li ions almost behave as a capacitor, the source impedance numbers are quite good.

 

[CWatters]

As I recall there were some electric powered busses in Singapore (?) that use supercaps in combination with other batteries. I'll try and find more info.

 

[Borek]

If the batteries are connected in parallel to capacitors, capacitors are basically all the time fully loaded from batteries, aren't they? How are they going to absorb any energy then? There is a bit of margin as the voltage drops under the load, but I still don't see how it is going to be in any way efficient without some kind of additional circuitry (basically I am saying the same thing @sophiecentaur said).

 

[CWatters]

Perhaps this is of interest..

"Combination of parallel connected super capacitor & battery for enhancing battery life"

https://ieeexplore.ieee.org/document/7877555

Same subject..

http://www.mouser.com/pdfDocs/PowerStor_SuperCaps_and_Batteries_White_Paper.pdf

 

[NTL2009]

Well I can' think of any other way you would run both batteries and capacitor energy storage together to feed the same DC bus for a motor drive. I mean you could link the caps with bi directional DC-DC to allow you to use the full voltage range of the cap, but that's likely more complicated than its worth.

I was just answering the OP question by showing how huge the cap bank would have to be even be noticeable, hence demonstrating that it doesn't make sense!

Yes, to use the super-caps efficiently, a separate converter is needed. As you say, this adds complexity, and the apparent positives of super-caps fades away pretty fast when all the realities are factored in. Both the size/cost and complexity. But there are some applications where they make a lot of sense, and costs an capabilities are improving.

 

[essenmein]

The motors are 2x 170kv 65amp each, the Vesc (motor controller) is a 200amp 10s. The bms is a 120amp continuous discharge, and the battery is an 18ah and capable of 150amp continuous discharge. I don't need or want my motors maxed out so I chose 50amps each bringing my total power required to 100amps continuous at any given time. I chose components that gave me head room for thermal efficiency and redundancy. Have I reached maximum efficiency because my battery can push 150a and I'm only asking for 100a? Would adding caps help reduce strain on the battery? I've ran out of room to add any more batteries so I was wondering if a few good caps could help.

Do you have a way of measuring/logging your DC current draw? Like do you literally never exceed 100A from the battery ever during use or are there maybe short bursts (few sec) of a 4-5 hundred amps or more during accel or regen? 100A from 10s pack is only ~4kW, which is not much.

 

[OmCheeto]

A 10Ahr 36V nominal battery stores ~1.3MJ, to have a capacitor that stores one tenth that (130kJ) over the same voltage range is about 300F

From my calculations, based on the "regen" part of thread title, I'd say a nominal energy requirement for the capacitors would be about 4000 joules.

1/2 x 84 kg x (9.7 m/s)² ≈ 4000 joules​

This assumes a CMS is installed. (Capacitor Maintenance System)

is only ~4kW, which is not much

For a bicycle? According to this website, 750 watts is the maximum allowable.

4000 watts ≈ 5 hp

 

[OmCheeto]

There would be little point in just connecting battery and capacitor in parallel.

I was in total agreement with your statement until I ran across the following at wiki; "...hybrid drive of up to 200 kW input and output power using "superbatteries" made with batteries and supercapacitors connected in parallel..."

Of course, being wiki, I googled for more information, FOR HOURS, with little luck.
I'm guessing now, from my research, and what nearly everyone else in the thread has stated, that the system was a bit more complicated than just "batteries and supercapacitors connected in parallel".

I also checked their "KERS" page, and found; "Experimental Machinist Douglas Goncz connected three ultracapacitor packs on an electric hub equipped recumbent bicycle in series/parallel with a 4PDT toggle switch in 2007 and described the resulting MObile Experimental Physics Educational Demonstrator (MOEPED) and its 19 kJ "electric flywheel" in a newsgroup posting to the moderated newsgroup sci.physics.research dated 11/9 of that year titled "MOEPED Update" and in other newsgroup threads under the keyword "MOEPED". In parallel, the packs were a regenerative brake; in series, they were a boost source. The vehicle remained in use so equipped until 2010. It was surplus disposed in 2013."

I may have to contact Mr. Goncz directly, and see if he has a block diagram of the system, as I have no idea how to access "newsgroups" any longer.
I did find him on "Google Groups", but he has been a VERY prolific experimenter, since at least 1999, and it's just too much to wade through.

hmmm...

(google, google, google)

Ah ha! I think this may be the original post I was looking for.
I will have to search some more, and find out why he disposed of his bicycle in 2013.

 

[essenmein]

From my calculations, based on the "regen" part of thread title, I'd say a nominal energy requirement for the capacitors would be about 4000 joules.

1/2 x 84 kg x (9.7 m/s)² ≈ 4000 joules​

This assumes a CMS is installed. (Capacitor Maintenance System)

For a bicycle? According to this website, 750 watts is the maximum allowable.

4000 watts ≈ 5 hp

Where do you get 84kg, 9.7ms, or the bicycle part from? I read this thread 3 times just now, and I'm pretty sure did not see those mentioned?

 

[OmCheeto]

Where do you get 84kg, 9.7ms, or the bicycle part from? I read this thread 3 times just now, and I'm pretty sure did not see those mentioned?

84 kg = me and my bicycle (Empirical data!)
9.7 m/s came from some website and my maths: fast roadies: 9.7 m/s = 21.7 mph = 35 km/h (real worldish stuff)
"bicycle" was interpolated from the given data (guessing we're not talking about an automobile )

 

[sophiecentaur]

modern Li ions almost behave as a capacitor,

The V/C characteristic of a Capacitor is a constant slope (the Capacitance. For a Lion battery, the drop in volts from near full to dying is pretty much a horizontal line. From full to usably empty, the battery volts only drop by around 10%. I can't see the similarity there.
I could imagine that, if weight is not a problem, with the battery fully charged, carrying some Capacitance could give you some advantage with acceleration for a short burst but, with the best will in the world, you can only get 10% of the charge in the Capacitor. But I don't see a problem with including some smart switching in a design that's obviously going to be a bit pricey in any case. (I'm only arguing against the simple idea of parallel connection.)
One point in favour of the SuperCapacitor is that the actual Energy capacity is all at the top of its voltage range ( Energy = (CV²/2) so you don't have to nearly discharge it. Any installation with SuperCs in it is going to have some fancy switching control in any case if the user is going to have any idea what the accelerator pedal will do for him.

 

[RedBaron]

Lots of great info everyone. I'm blown away by the responses and the amount of information.

My max amp draw is capped to 100A in the programming of the motor controller So there shouldn't be any bursts going on. I'm 155lbs. I was interested in adding super or regular caps because I do a lot of stop and starts on the route I'll be taking. Here is a super rough sketch of my set up. I would be adding a bank of caps inline with the leads going to the motor controller from the bmsP- and battery+. I have heard of people doing this but wasn't to sure how it works exactly or if it would be of any benefit even though my battery is theoretically capable of 150a sustained.

 

[sophiecentaur]

Without some serious experimenting and measuring, you cannot tell just what the arrangement is achieving. Without thinking too deeply about it, I would worry about any long term harm that the capacitors could be doing to the batteries. Lions need to be charged right.

 

[essenmein]

The V/C characteristic of a Capacitor is a constant slope (the Capacitance. For a Lion battery, the drop in volts from near full to dying is pretty much a horizontal line. From full to usably empty, the battery volts only drop by around 10%. I can't see the similarity there.
I could imagine that, if weight is not a problem, with the battery fully charged, carrying some Capacitance could give you some advantage with acceleration for a short burst but, with the best will in the world, you can only get 10% of the charge in the Capacitor. But I don't see a problem with including some smart switching in a design that's obviously going to be a bit pricey in any case. (I'm only arguing against the simple idea of parallel connection.)
One point in favour of the SuperCapacitor is that the actual Energy capacity is all at the top of its voltage range ( Energy = (CV²/2) so you don't have to nearly discharge it. Any installation with SuperCs in it is going to have some fancy switching control in any case if the user is going to have any idea what the accelerator pedal will do for him.

Clearly a Li ion is not actually a capacitor, but their dynamic impedance is not that different during short current pulses where the V is assumed to be more or less constant, after all both are used as "short term" voltage sources. Now I'm comparing large electrolytics to Li ions, not something like a film or ceramic where you can get v low ESL.

Also its quite reasonable to model a Li ion as a capacitor with the right esr/esl and the capacitance value being determined by the energy stored in the battery between Vfull and Vempy.

Paralleling capacitors to a battery for short bursts would make sense if the battery source impedance is "slow" vs your application or you have very large inrush currents that might damage them (eg the starter batteries in the links above), but you would have to measure how much the volt dip is during said pulse at the inverter terminals to determine if this has any value. Keep in mind that any motor drive is already going to have some pretty high quality capacitors on its internal DC bus to deliver the xkHz PWM current and keep that HF current local to the inverter. Basically for transient current its an impedance matching game and the capacitors would have to approach the source impedance of the batteries for them to have any worth while effect.

To me the OP is not pushing his batteries (rated 150A and only pulling 100A) and unless the voltage dip is significant at the inverter terminals a capacitor would likely have minimal effect for transients and would need to be huge to provide noticeable energy storage increase.

IMO determine if there is even a problem with the set up as is by measuring before adding more stuff to it, if its a hobby and you just want to have the caps then fine but in any sort of product development there'd have to be a pretty compelling case to add them to warrant the cost.

 

[essenmein]

Without some serious experimenting and measuring, you cannot tell just what the arrangement is achieving. Without thinking too deeply about it, I would worry about any long term harm that the capacitors could be doing to the batteries. Lions need to be charged right.

No problem with charging the li ion with parallel caps, battery management systems monitor cell voltage and a parallel cap won't affect the end voltage level, just the time taken to get there.

 

[sophiecentaur]

No problem with charging the li ion with parallel caps, battery management systems monitor cell voltage and a parallel cap won't affect the end voltage level, just the time taken to get there.

But where would be the advantage in using a Capacitor compared with just a bigger battery bank? Do you not want to get some benefit from the fact that excess motor volts can be used to put charge into the capacitor without invoking a regulator to protect the battery? The QV slope of the capacitor will severely limit its usefulness unless you can manage the Energy. Connecting a battery straight across it prevent the capacitor volts from going high enough for it to store excess energy.
This may be less harmful than I thought but it still doesn't allow the capacitor to be used to advantage.
I'm assuming that there will be some cost-benefit consideration when the value of the capacitor is chosen. It's true that a big enough capacitor could store loads of energy at battery volts but the 'square law' relationship means that, say 10% higher volts on it than the battery volts would mean 21% extra stored energy in it. I would have though it would be worth going for
The 'equivalent capacity' of a battery is enormous, based on Q/V, if the Volts don't change much.
"just the time taken to get there". If they are in parallel, what will make the volts increase faster?

There is nothing wrong with contemplating a hybrid system if you are prepared to make full use of the two storage media. One thing that would be useful would be to charge the battery from the C when the system is not being used. That would involve a boost circuit - to augment the buck circuit for when the C is highly charged.Unless you are prepared for some testing, you would have no actual quantitative idea about whether or not your modification is doing what it could to should.

 

[essenmein]

But where would be the advantage in using a Capacitor compared with just a bigger battery bank?

IMO there isn't one unless you have a very specific problem to solve.