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Capacitors and regenerative braking

  1. Nov 9, 2018 #1
    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 lets 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.
     
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  3. Nov 9, 2018 #2

    phinds

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    You already HAVE regenerative braking. Why do you think adding additional storage will matter? Do you think your current storage is insufficient?
     
  4. Nov 9, 2018 #3

    berkeman

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    Welcome to the PF. :smile:

    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.
     
  5. Nov 9, 2018 #4
    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.
     
  6. Nov 9, 2018 #5
    Also I dont have a good way of providing a schematic. But I'll work on it.
     
  7. Nov 9, 2018 #6

    berkeman

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    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. :smile:
     
  8. Nov 10, 2018 #7

    davenn

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    where are you getting 170,000 V from ?
     
  9. Nov 10, 2018 #8

    Borek

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    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 :smile:
     
  10. Nov 10, 2018 #9
    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.
     
  11. Nov 10, 2018 #10

    davenn

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

    edit .... so my next Q is ...
    is this even a valid/practical value for what the OP wants to achieve ?
     
  12. Nov 10, 2018 #11
  13. Nov 11, 2018 #12

    Borek

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    Oops, my bad.
     
  14. Nov 12, 2018 at 3:50 AM #13

    sophiecentaur

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    A sketch with a sharp pencil and a photo is usually possible. Not the best but it can carry the message.
     
  15. Nov 12, 2018 at 3:56 AM #14

    sophiecentaur

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    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.
     
    Last edited: Nov 12, 2018 at 4:04 AM
  16. Nov 13, 2018 at 2:14 PM #15
    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, thats 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.
     
    Last edited: Nov 13, 2018 at 2:28 PM
  17. Nov 13, 2018 at 2:50 PM #16

    sophiecentaur

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    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.
     
  18. Nov 13, 2018 at 3:25 PM #17
    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!
     
  19. Nov 13, 2018 at 3:50 PM #18

    sophiecentaur

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    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.
     
  20. Nov 13, 2018 at 4:03 PM #19
    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.
     
  21. Nov 13, 2018 at 4:06 PM #20

    CWatters

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    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.
     
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