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  1. #11
    Electric Warrior CaptainKlapton's Avatar
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    Nice! it's good to see calculators like this. I have been working on something similar in MatLab. It really helps to be able to get all the info in one place, catered to your individual needs. My MatLab script will calculate approximate pack resistance and heating. I am trying to make it better for how dynamic cell resistance is, especially with temperature. Right now I am just using the formula:
    Rpack=(Rcell/Parallel)*Series
    "Never let schooling interfere with your education."
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  2. #12
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    130 kgs rather than 350 kgs :-)

    Assuming I've understood things right: Voltage under load = total pack voltage - (current x resistance)

    Of course the key thing to remember is that the greater the resistance the more heat that will be generated from the pack!

    Not quite sure why you are dividing by the number of Parallel packs, surely it is just a factor of the number is series?

    Rpack=(Rcell/Parallel)*Series
    Last edited by Mike Edwards; 18 July 2013 at 1723.

  3. #13
    Electric Warrior CaptainKlapton's Avatar
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    Quote Originally Posted by Mike Edwards View Post

    Not quite sure why you are dividing by the number of Parallel packs, surely it is just a factor of the number is series?

    Rpack=(Rcell/Parallel)*Series
    I should have specified, I meant series and parallel cells. I believe that to find the equivalent resistance, all of the normal circuits rules for resistors apply. Please correct me if I am wrong on this, I am still learning. If they do apply then dividing by the number of parallel cells is the same as 1/[(1/R1)+(1/R2)...] just shorter because I assumed all the cells had the same resistance value.
    "Never let schooling interfere with your education."
    -Mark Twain

  4. #14
    Member PaulWay's Avatar
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    130kg. Ah, sorry Boy, that's a misread :-)

    Logically, it hasn't given you any >25Ah cells because they weigh too much. It's easy to see how this works - 130 kilograms divided by 190 cells (for LiPo) to produce 700V gives about 680g per cell. You can't make a pack out of heavier cells without it going over 130 kilos. Running my eye down http://mabula.tangram.dnsalias.net/b...ell_info?s=wgt I see that the Dow Kokam XALT HE 25 comes in at less than 680g, and it does appear on http://mabula.tangram.dnsalias.net/b...700&weight=130, but there aren't any cells greater than 25Ah that are less than 680g per cell.

    Does that make sense?

    Please feel free to play around with increasing the weight to see when you get the sweet spot of cells you want.

    What kind of controller and motor are you looking at there, Mike?

    Have fun,

    Paul

  5. #15
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    Motor wise we are looking at the Yasa 400 and the Sevcon controller coupled to our own ECU.

    Can't see the XALT HE 31 (just about right) or XALT HP 31 (too heavy) in your results but I guess it depends on how close to 700V you want to go.

    189 x XALT31 @ 705g each = 699.3V and 133.25Kg but a few less cells and you are under the weight limit and still pretty high on the pack kWh capacity.

    As for resistance, I was under the impression that it was simply: cell count in series x resistance. I can't see why taking parallel packs in to consideration would reduce the resistance.

    I would calculate the voltage under load as:

    voltage = nominal voltage - (peak current x cell resistance x number cells in series)

    Not forgetting to convert cell resistance in to ohms rather than milliohms first.

    Would welcome some input in to the calculation for heat generation...

    The basic I2R calculation should work okay:

    heat (watts) = peak current x peak current x cell resistance x number cells in series

    Where peak current = cell current x peak 'C' rating x packs in parallel
    Last edited by Mike Edwards; 19 July 2013 at 0506.

  6. #16
    Member PaulWay's Avatar
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    Quote Originally Posted by Mike Edwards View Post
    Motor wise we are looking at the Yasa 400 and the Sevcon controller coupled to our own ECU.

    Can't see the XALT HE 31 (just about right) or XALT HP 31 (too heavy) in your results but I guess it depends on how close to 700V you want to go.

    189 x XALT31 @ 705g each = 699.3V and 133.25Kg but a few less cells and you are under the weight limit and still pretty high on the pack kWh capacity.
    Yeah, that's the way with computers - they take things literally If you say "700 volts" it'll assume that 699.99 is too low. If you say 130kgs, then 130.01 is too much. Sometimes it helps to broaden your search out a bit if you think that there are options not presented that should be there. I don't really want to make it try to treat the numbers given as 'guidelines' because then it'd be a bit too confusing, IMO. Sometimes you want to say "cannot take one more kilo than this".

    Hmmm - maybe a checkbox that says 'include near misses' and invisibly adds 5% to those numbers... (highlighting the ones that still made it within the limits).

    Re resistance - if you have two strings in parallel, the resistance will be halved - that's basic electrical ciruitry. I'd have to run some numbers and see if they actually make sense.

    But I wonder if that's really useful at all - after all, the voltage sag at full load is going to be quite different across the discharge range of the battery. Load a LiFePO4 cell up at 3.65 volts and it'll sag in a quite different way to when it's at 2.8 volts...

    Anyway, I'm glad the pack calculator is working for you :-)

    Have fun,

    Paul

  7. #17
    Senior Member jonescg's Avatar
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    Oh I should add - The C040 cells from EIG are not in production yet, so don't base realistic assumptions on this cell being available with ease. The F014 cells are also only available in 5000 cell orders as they will restart a production run if demand is there. C020 cells are good to go however.

  8. #18
    Member PaulWay's Avatar
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    Hi all,

    Just a quick note to say I've updated Rebirth Auto and added EV Motor Werks and EVTV Motor Verks; Rebirth and EVTV have the new CALB cells, EV Motor Werks says "CALB" all over the page but when you look into the specs they're still using the old Sky Energy cells.

    Hope this helps,

    Paul

  9. #19
    Senior Member jonescg's Avatar
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    Oh if you like you can update the price of C020 cells from EV-Power to $60 each, although now that the dollar has dropped again we might have to make that $65...

  10. #20
    Member PaulWay's Avatar
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    Righto, have made them $65 + $6.50 GST = $71.50. If that price included GST, let me know and I'll change it

    Have fun,

    Paul

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