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Thread: Can i calculate the battery side current?

              
   
   
  1. #21
    Not to be taken seriously DaveAK's Avatar
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    Quote Originally Posted by __Tango View Post
    What hall effect sensor are you using? I'm looking into getting one as well. Dimitri from MiniBMS uses a Tamura L01Z, but i don't think that will fit my 1/0 gauge.

    Ooooohhh...cell connection. I hadn't thought of that.

    Actually, that's going off in a slightly different direction that I'm thinking for mine, which may be more BMS like in the future...
    Hi, I'm Dave. I'm a dataholic.

    I ended up getting the Honeywell CSLA2EL from Mouser. There was another cheaper one that I was going to get originally but they were having supply issues. I probably should have just waited because I haven't got around to using it yet.

  2. #22
    Señor Member podolefsky's Avatar
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    Quote Originally Posted by lugnut View Post
    This sum should be zero. In fact, if it is not zero, you have a ground fault. At least that is the way sine wave 3 phase VFDs behave.
    Yes, if you sum the instantaneous voltage or current for three phase, you will get a sum of zero. But the thing you want for backing out the battery current is motor side power. P = I*V. For three phase it's a little complicated, but not that much.

    The reason the power isn't zero is because the voltage and current have the same sign, so I*V will always be positive.

    So with a BLDC controller, it depends on what the controller is spitting out for the voltage and current on each phase. I doubt it's instantaneous, the data stream for that would be way too dense. I'm guessing it's RMS, which *I think* means you should be able to do P = I*V for each phase and just add them up.

    For BLDC, it's going to be 2x the power for any one phase (at any given time, two phases are on and one phase is off). So P_total = 2*I*V for one phase. Then do I_battery = P_total / V_battery

    This image shows how V and I are lined up (in phase) for BLDC. (here, solid lines are voltage (e) and dashed are current (i))

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  3. #23
    Empulse R #24 frodus's Avatar
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    Depends on the BLDC....

    Mark, how does kelly do the BLDC? Is it a trapezoidal wave? sawtooth? or pure sinusoidal?

  4. #24
    Señor Member podolefsky's Avatar
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    Kelly BLDC is trapezoidal. Sinusoidal is generally referred to as AC (or PMAC), as in the Sevcon Gen4 controllers. (I shouldn't say generally, people use BLDC to refer to everything...but if we're being picky)

    I only know this because I asked John Fiorenza about running the ME0913 with the Kelly, and he said the trapezoidal would be noisier than the sinewave from the Sevcon.
    Last edited by podolefsky; 17 June 2011 at 1402.
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  5. #25
    Señor Member podolefsky's Avatar
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    Quote Originally Posted by markcycle View Post
    It's very difficult to calculate the RMS of a non sinusoidal or other well known AC waveform (such as square or triangle). The AC peaks can be as high as 5 times the DC current. On the controller I'm working on the AC peak is as much as 2.5 times or more the DC current. The RMS or average power is always a little less than the DC power or equal to when including the controller losses
    RMS values for sine, square, and sawtooth are simple:

    http://en.wikipedia.org/wiki/Root_mean_square

    And sorry for being picky...but, RMS refers to voltage or current. There's no such thing as RMS power, just average power (usually averaged over time). For 3-phase, both instantaneous and average power are constant and equal.

    You are right that average power out has to be equal to DC power in (minus efficiency losses).

    Just clarifying - not trying to be a d!ck.
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  6. #26
    Senior Member __Tango's Avatar
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    Noah. Thanks. That was a great explanation that my pea brain can kind of understand....er...maybe.

    I have data from a ride today. it's in excel spreadsheets at the moment. The first one is a summary of the whole ride at one sample per second.

    http://electriceptor.files.wordpress...19-summary.xls

    It's got all of the data from GPS and Canbus (most everything is labeled including units up till column O. At that point, look at the kelly docs (http://kellycontroller.com/mot/downl...UserManual.pdf) for the unit informatino (for CCP_A2D_BATCH_READ1, CCP_A2D_BATCH_READ2, CCP_MONITOR1, and CCP_MONITOR2)

    The second and third are approx 1 second dumps of data as fast as i can get for the motor side current and voltage (which currently is about 150 samples/sec).



    I was trying to calculate power based on this information and noah's explanation, can i take the max of the I and V for a timeslice, multiply it by 2 to get power?
    Last edited by __Tango; 17 June 2011 at 1442. Reason: formatting
    EVs: 2007 Zapino Scooter, Honda VF500 Conversion (electriceptor.wordpress.com), Red Nissan LEAF SL (Arrived!)

  7. #27
    Senior Member __Tango's Avatar
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    Quote Originally Posted by DaveAK View Post
    Hi, I'm Dave. I'm a dataholic.
    Ditto. I'm in a 12 step program ("Hi Dave!") and am working on it.

    Quote Originally Posted by DaveAK View Post
    I ended up getting the Honeywell CSLA2EL from Mouser. There was another cheaper one that I was going to get originally but they were having supply issues. I probably should have just waited because I haven't got around to using it yet.
    What was the cheaper one you were considering?
    EVs: 2007 Zapino Scooter, Honda VF500 Conversion (electriceptor.wordpress.com), Red Nissan LEAF SL (Arrived!)

  8. #28
    Empulse R #24 frodus's Avatar
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    But I wonder why Kelly can't add that to the firmware? I mean, isn't battery current REALLY important for us EV'rs? I guess they think the BMS is gonna do that.... or some other sensor.

    They're already calculating current on the output (they have to in order to current limit), so why not just store those currents and add them and spit that out on canbus? It's already IN the software (partially), just not implemented.

  9. #29
    Señor Member podolefsky's Avatar
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    Quote Originally Posted by frodus View Post
    But I wonder why Kelly can't add that to the firmware? I mean, isn't battery current REALLY important for us EV'rs? I guess they think the BMS is gonna do that.... or some other sensor.

    They're already calculating current on the output (they have to in order to current limit), so why not just store those currents and add them and spit that out on canbus? It's already IN the software (partially), just not implemented.
    They probably can, they just didn't. My Alltrax spits out battery current, motor current, and battery voltage...but not motor voltage (but you can get that from the throttle %).

    Speaking of things they should do .... that HSR controller I was looking at (that does series motor regen) has separate motor and battery current limits. I think most of the other Kelly's just have motor. I don't know why, it seems like a really useful feature. it will let you protect your batteries without limiting motor current at low RPM (when motor current > battery).
    - Noah Podolefsky -
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  10. #30
    Senior Member markcycle's Avatar
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    great data Alex
    I plotted phase current VS battery voltage just to get a feeling for voltage drop under load
    Trying to Change the old school ways of doing things
    But I never Never try to break the laws of Physics
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