FAQ: LiPo 101 in flight. Props, ESCs, Motors, and Batteries. Oh my.

Discussion in 'Walkera Talk' started by bjr981s, Oct 25, 2018.

  1. bjr981s

    bjr981s Moderator Staff Member

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    This is a reference thread for LiPo batteries and the drive trains of Electric Models.

    LiPo 101 in flight.

    • Number of cells is the number of Individual 3.7V (Nominal Voltage) cells in a pack. Cells x 3.7V = Nominal pack voltage. e.g. 3S = 11.1V
    • C Value is the maximum current that a battery can provide before it voltage sags. Max A = C x AmpHours e.g. 2200mah 25C = 25 x 2.2 = 55 Amps. Note a battery as it discharges reduces the max current. e.g. a half flat, 50% charge is only 2200mah x .5 = 1100mah. So formula is actually Charge % x C x mah rating. So a dead flat battery = 0 x 25 x 2.200 = 0 amps max.
    • Only 60% of the mah rating of a battery is usable before the max current becomes critical. Also discharging below 40% charge damages the cell and reduces the number of lifetime cycles. Never discharge below 3.5V per cell quiesced. (after load removed)
    • Power in watt hours is the pack voltage x the current being drawn. The more cells in a pack the more power it can deliver at the same current output. e.g. Our 2200mah 3s at 5 amps = 3 x 4.2V x 5A = 189Watts. A 2200mah 6S will be 6 x 4.2V x 5A = 378W.
    • The number of cells you can use in a flight pack is dependant on the Motor specifications and the ESC you are using.
    • The motor will have a KV rating that defines the number of revs per volt. e.g. A 1000 KV specification will rev the motor at cell count x 4.2 (max Charged voltage) e.g. with our 2200 3S that will be 1000 x 4.2 x 3 = 12,600 revs. ( this is max and does not include the power required to spin the Propeller.)
    • The amount of current drawn through the ESC is the current the motor will draw attempting to match its KV rating. This is dependant on the drag the propeller is providing. The maximum current a motor will draw is the stalled current of the motor. I.E. stopping the motor from turning. An electric motor has max torque at 0 revs.
    • Exceeding the maximum rev specification, and or the max power rating will damage the motor. Exceeding the max current of the ESC will damage the ESC.
    • The propeller specification (drag) will actually define the amps of current drawn. Length and Pitch. The longer the prop and the coarser the pitch times the number of blades the more drag. Also the more air is drawn by the blades. The most efficient prop is a single blade for speed, the more blades for power.
    • Propeller, Motor, ESC, and battery specification is the drive train specification and must be matched to the Weight (Drone) or Wing loading (Fixed Wing) to achieve maximum efficiency, no damage and flight times. The battery specification in Max Current must be sufficient to cover the max stress imposed as a transient for instantaneous throttle up from 0 to full for the specified drive train.
    • All electrical circuit specifications are dependant on the temperature of the components. What would normally be fixed in Ohms Law in flight V=IR (resistance) is not, however. The ESC, battery, motor are all components that generate heat as they dispose of and consume energy. The resistance of these components increases with the temperature. So you must also ensure that you do not exceed the temperature specifications of the components.

    Manufacturers tune their BNF and RTF models to the specifications including the flight parameters. Changing any one of these will change the flight characteristics. and potentially generate motor or ESC damage.

    For do it yourself construction (or changing drive train components) you need to calculate and measure these parameters.

    You need the correct tools.

    This will cover the majority of cases. I use 2 in parallel for analysing my Scratch and kit built fixed wings for up to 2 engines from single battery.

    Turnigy Thrust Stand and Power Analyser v3

    Cheers Brian.

    p.s. 5 minute flight time is about the correct balance for an electric powered fixed wing.

    Last edited: Nov 29, 2018

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