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Thread: Battery Time!

              
   
   
  1. #1
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    Battery Time!

    So I'm looking for a battery pack with these specs:

    System Voltage: 96V
    Peak Current: 600A for up to a minute
    Costant Current: 200A to stay at freeway speeds
    Costant Energy: 6.174 kWh

    This is the best deal I've found for anything lithium (still haven't seen any deep cycle lead or nickel that were worth the hassle):
    http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html

    Larger packs have too hard a voltage to stack above, but still close to, 96V, and moving up in capacity is much more expensive than buying several of the smaller cells. I was looking at buying 81 of them (9 strings of 9, can pump the max 600A for 10 sec.) to test my controls and motor with, then buying more as time went on (just to extend range).

    I hear lots of nasty stories about lithium polymer batteries though, so I'd like to hear form anyone hear who uses them successfully. I'd like to hear what chargers and battery management techniques you use, but even better would be some technical documentation with balancing topologies and whatnot so I can understand them better and hopefully skip having to buy a BMS.

    Thanks
    Last edited by s33dless; 05 January 2014 at 1916. Reason: typo

  2. #2
    Senior Member Skeezmour's Avatar
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    If you run those without some type of monitoring at least you will wind up having problems. There are many lower cost options that people here have used that may work for you. Do some searching through the forum. I will say if you need 200a to hold freeway speed at 96v you have some high freeway speeds. Most of the electric cars I have built don't need 20kw to maintain 60+mph on level freeway.

  3. #3
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    Quote Originally Posted by Skeezmour View Post
    If you run those without some type of monitoring at least you will wind up having problems. There are many lower cost options that people here have used that may work for you. Do some searching through the forum. I will say if you need 200a to hold freeway speed at 96v you have some high freeway speeds. Most of the electric cars I have built don't need 20kw to maintain 60+mph on level freeway.
    I'm cruising through the forums as we speak, it's just an extremely slow way to extract info. 4-5 relevant posts per thread, but that's nobody's fault...the topic just isn't mine.

    I might have donked up some of my math, I'll double-check.

    EDIT: Forget might have, try did.
    My power calculations don't seem to agree:
    P(t) = F * v = 17.95 kW
    P(t) = t * w = 2.856 kW

    I think I have a problem with my force calculations, I'll have to hunt for it. Might have just used the wrong equation somewhere.

    EDIT 2:
    Correction, I was using the wrong "w" in my check earlier, they both come out to 17.95 kW. I'm willing to bet I have a unit conversion error somewhere.

    EDIT 3:
    Can't find my mistake, I'll take this as a sign to go to bed. Basically, I need enough torque to be able to beat aerodynamic drag and rolling resistance to keep speeds 75+:
    calc_drag_force_N = ((1/2) * mass_density * CdA* ((velocity_kmh*1000/3600)^2))

    calc_drag_force_N = ((1/2) * 1.225 * .7 * (((120.701)*1000/3600)^2)) = 482 N

    calc_roll_force_N = c_rr * gross_weight_kg * g

    calc_roll_force_N = .015 * (272.155 + 90.175) * 9.8 = 53.34 N

    Total force to overcome = 535.3 N
    So the moment we need on the motor shaft itself will be:

    calc_running_torque_Nm = (calc_running_force_N * (wheel_radius_mm/1000))/(axle_ratio * transmission_efficiency)

    calc_running_torque_Nm = (535.3) * (215.9/1000))/(3.7 * .9) = 34.71 Nm

    The power I get from the mechanical calculations is what I use to calculate the required pack parameters, so 17945W / 96V = 187A. But I know my motor has a torque constant of .15 Nm/A, which tells me I need 231.4A to get 34.71 Nm.

    It's starting to look like a snarl, I think I need to give up on this for today and get some sleep...
    Last edited by s33dless; 05 January 2014 at 2128.

  4. #4
    Seņor Member podolefsky's Avatar
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    Check out the Elmoto spreadsheet. Will save you tons of work. Enter your data and motor torque curve and it does the rest, and the results are pretty accurate. Spreadsheet says about 5 kW to go 60mph on a 450 lb, naked bike - that's very close to what I get in the real world.

    https://www.elmoto.net/showthread.ph...2-0-Data-sheet


    The link in the first post is broken, so I put it up on my site:

    http://www.colorado.edu/physics/Educ...Elmoto2.18.xls
    Last edited by podolefsky; 06 January 2014 at 1335.
    - Noah Podolefsky -
    The GSX-E

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  6. #5
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    Thanks, I'll have a look at it at work in the morning (no excel on my home machines, don't use it). I'm coding all these calculations, so it's really no work at all, but I'll double-check my models against whatever equations they have in the spreadsheet. I ran my block with those input parameters and got 8.274 kW, which is very oddly close.

    EDIT: Gah, looks like my math is really donked up.

    ANOTHER EDIT:
    Nope, looks like I'm in agreement with the ElMoto worksheet. Both say 17.95 kW for 120 km/h. I used the following parameters:
    1)adjust weight so gross ("Total Mass") is 600 lbs
    2) 94% efficiency, 5000 RPM
    3) 3.7 Gear Ratio
    4)g = 9.8
    5) area = 1m^2, drag coeff. = .7 (to get CdA of .7)
    7) air density 1.225
    8) drag coeff .015
    9)no hill, no wind, 100% controller eff., 550A
    Last edited by s33dless; 06 January 2014 at 0959. Reason: Checked math

  7. #6
    Seņor Member podolefsky's Avatar
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    Hmmm...when I look at the spreadsheet, I see about 9 kW for 120 kpm (75 mph). It could be a bit more, but not 18 kW.
    - Noah Podolefsky -
    The GSX-E

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  9. #7
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    Then I'm doing something very, very wrong in my inputs.
    Attached are some screenshots, thanks a billion for helping me out.
    screen1.jpg
    screen2.png

  10. #8
    Seņor Member podolefsky's Avatar
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    Sorry, can't read from the low res image you posted.

    I'd suggest using the default parameters in the spreadsheet. Then just change the "rolling chassis" to get the weight you want, and change "number of batteries" to 32 (for 96V). The power calculations are independent of the motor torque curve, so you don't need to mess with that for now.

    Then go down to the green box labeled "Range" and look toward the bottom. Enter the speeds you want and it will tell you the current, multiply current * voltage (just use 96V) and you'll have a good estimate of power required.

    Defaults are for a 300lb bike, so your 600lb bike will have a little higher power requirements.
    - Noah Podolefsky -
    The GSX-E

  11. #9
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    Ack, I didn't realize it resized.

    https://docs.google.com/file/d/0B56s...Rm8/edit?pli=1

    That link has a full res version of the picture, you can zoom at will.

    I would very much like to be able to just plug numbers in and run, but I need fine control over these variables because I'm building the controls and power system from scratch. I'm not too familiar with all this mechanical modelling, hence all the trouble, but once I settle this (soon by the looks of it, I'm proud for at least picking the right stuff to model with no ME help!) stuff it should all be downhill (Circuits are my forte and I find power electronics and controls to be fun). In fact, the only thing the worksheet has that I don't is wind speed adjustment.

  12. #10
    Seņor Member podolefsky's Avatar
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    Sorry, I just noticed that the current in the "Range" box changes with gear ratio, so it's not what I thought it was (maybe it's motor current?). Battery current should be almost entirely independent of gear ratio.

    I think the reason you're getting a very high power requirement is having a high CdA. I believe 0.7 is about as high as a motorcycle gets, like sitting straight upright on a big dual sport. The spreadsheet default is ~0.29, which is on the low end, more like tucked on a sportbike. Putting it up to about 0.4 gives results pretty close to what I see in the real world on my mid-size naked bike.
    - Noah Podolefsky -
    The GSX-E

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