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Do you limit your flight time by Volts or Amps

Discussion in 'Cinestar Misc' started by Tyler Olson, May 8, 2013.

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Do you watch the volts amps or both to limit your flying time

  1. Volts Primarily

    84.2%
  2. Amps Mainly

    0 vote(s)
    0.0%
  3. Whichever shows 20% first

    15.8%
  1. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    If I were to be deposed in litigation, my answer would be: "I set my low battery voltage threshold very conservatively at 14.4 volts which, for a 4S battery, allows for an appropriate reserve of flying time under normal circumstances. I do this based on what I believe is an accepted standard within the Cinestar community. I take flight safety seriously, and this is an indication of how seriously I take it."

    Hope this helps.
    Andy.
     
  2. Colin Snow

    Colin Snow Active Member

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    It does. Thanks much Andy.

    Colin
     
  3. Graydon Tranquilla

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    Power meters provide the most accurate picture of remaining battery capacity. P = V * I

    Volts is simple and instantaneous to measure without drawing any current.
    Amps measurement requires momentary shunting which draws down battery capacity.
    And amp measurement also requires a bit of time such that amp and power messurements cannot be done in realtime.

    Volt level may be a very indirect method for determining remaining battery capacity but it is fast.
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Although there might be an argument that measuring the voltage of the LiPo under load -- and monitoring every 15 seconds during a flight gives a reasonable indication of the state of the battery given a normal discharge curve.

    The MK Brushless Controllers determine the current to the motor by measuring a voltage drop across a small shunt through which the current is flowing, thus inferring the current based on I = V/R, where V is determined by an ADC on the CPU, and R is a known value (of the shunt).

    Can one actually measure current?

    My physics knowledge is probably out of date, but there seems to be only two ways: voltage drop across a resistance, or voltage emitted from a biased Hall-effect sensor like the Honeywell CSLA2CD http://www.amazon.com/CSLA2CD-Honeywell-Current-Transducer-mounting/dp/B006DD0WCO? So in both cases you're measuring the effect of current, not the current itself? Is this still the case?

    Andy.
     
  5. Shaun Stanton

    Shaun Stanton Active Member

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    I agree that is the basic way of analyzing current pull. There might be some transient response from the current to the motors since the motors are inductor coils but I would believe this to be negligible. The battery feels what is being pulled, that results in the voltage sag in the battery. I think you can get an overall idea of what the battery state is by knowing how much voltage is being dropped for the amount of current being pulled.

    I don't really find instantaneous current draw that useful other than SA on the performance of the system and to know if I might overstress something. I find that current used is sort of like a reverse fuel gauge. If I know I have a 8000mAh Battery and I have used 7000 mA in 7min I know that I should start thinking about landing because I probably have a minute left of flight before going below safe battery Voltage. Their is no exact science to it. I have found some batteries I can go further than others. I back up the current used with the voltage of the battery for the load. That gives me an idea if the battery is depleting earlier or losing its ability to handle the discharge demand.

    I find that I expect about a .5 to .7V sag in the battery at takeoff if the battery is new. As the battery starts getting old the sag starts getting larger to a point where in a climb it starts approaching th e20% mark. When I see this I will land and bench that battery.
     

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