Another Flight Time Thread

Discussion in 'Cinestar 8' started by Josh Conley, Apr 9, 2013.

  1. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    It's worth adding that the C rating, in terms of the discharge rate, is affected by the internal resistance of the battery -- one only has to imagine (only ONLY imagine this) connecting the two terminals of a LiPo together -- the amount of current will (at least for the first few milliseconds until something melts or ignites) be limited by the "resistance" of the circuit -- in this case, the internal resistance of the battery is all that stands in the way (well, plus the resistance of the wires).

    Based on some experiments I did, this resistance is typically down in the 25 - 38 milliOhms range -- which is why a LiPo can source such large currents.

    Heating is controlled by the square of the current as I recall my physics. Power = Volts x Volts / Resistance, I think.

    But don't forget -- in a system with X units each having a probability, p, of failure, the larger X is, then the larger p becomes. Ask any pilot with a twin engine rating!

    The probabilities work like this: A dual battery setup:
    1. Is twice as likely to develop problems with either battery.
    2. Is four times less likely to develop problems with both batteries.

    The kicker is that, the way the dual batteries are wired up, item 2. never gets to apply -- because if one battery fails, I suspect it is going to drag the other one down with it. So all you're left with is, with a dual battery, it's twice as likely to fail than a single battery.

    So in Dirty Harry's universe, the phrase is: "D'ya feel lucky today, Copter Pilot....?" :)

    What we really need to make dual batteries safer is an isolating circuit so that, when one battery fails, it is isolated from the second battery.

    Andy.
     
  2. Gary Kaplan

    Gary Kaplan New Member

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    Andy,

    Good point. You are entirely correct on both issues about the probability of failures increasing and the internal resistance related to discharge rate. My only purpose was to note that a 4S 8000mAH 4S2P battery pack is essentially (2) 4S 4000mAH packs in parallel inside one shrink wrap with the internal connections you can't see. In a nutshell my point was that is if in your system you have motors that want 30A and an ESC's rated at 30A then pilots will have less battery issues if they use a high capacity 65C/130C battery over a 30C/50C pack.

    Love your idea about the battery isolator! If you could figure out how to buffer the voltage and figure out how to switch you'll have one hell of a $ maker. Make it happen. I want one!

    Gary
     
  3. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    GaryK: Sorry....didn't mean to sound like I was contradicting you -- I was just trying to add the afterthought relating to dual batteries.

    Yeah, switching from 30-60 Amps at up to 24 volts would be nice. That's a 1.4 kilowatt switch. :) Given it's just for emergency use, it might be easier to use some kind of a one-time fusible link rather than silicon switch. But first you have to detect that a battery has failed, then blow the link to isolate the failed battery. That would be pretty easy....

    Andy.
     
  4. Gary Haynes

    Gary Haynes Administrator
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    Perhaps this is a very conservative approach. But the standard approach seems to be different. Using the example for the 5000mah 6S:

    Turnigy nano-tech 5000mah 6S 65~130C Lipo Pack: Spec.
    Capacity: 5000mAh
    Voltage: 6S1P / 6 Cell / 22.2V <--- Its (1) 6S pack
    Discharge: 65C Constant / 130C Burst <--- This is what you want. It can source 65A all day without breaking a sweat.

    The continuous AMP pull for this battery is really 5A x 65 = 325amps continuous. Formula, is Capacity in Amps x C rating = Continuous Safe Amp draw for that battery setup. Burst would be a whopping 650 amps. That's 81.25 amps per BL/ESC/Motor on a CS8.

    Most quads that folks are flying are drawing 50-70 amps. So if you want a conservative 100 amp continuous safety cushion with a 5000mah battery anything at or above a 20C rating should be adequate. Don't know if there is a downside other than cost to using a higher C rated battery. Think I asked that someplace once and never got a reply.
     

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