Voltage drop

I'm having a real boring issue with the hexacopter starts beeping and warning for low voltage as soon as I take off.
It doesn't warn when I take off without the camera gimbal attached.

My spec is:
Pixhawk with 3DR power module
Tarot 680 Pro
6x Sunnysky V3508 700kV
13 inch props
DSLR Gimbal with SToRM32
2x Multistar 5200mAh 4S 10C

I'm having the two batteries wired in parallel and that should add up to 20C discharge rate=able to deliver 5.2*20=104Amps.

Surely, my hexacopter can't draw this much and cause the batteries to drop so much in voltage?? Please help!

The voltage

j1uhdNs.png?1

The current
2jTT4fX.png?1

2015-10-18 17-58-52.log

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Replies

  • Hi everybody,

    I have exactly the same problem, PM voltage drop even on hover after a few seconds, on fully charged battery. Separately installed power meter shows consistent voltage of around 15.5 V in hover, but Pixhawk triggers Batt Failsafe, because the voltage reported by the PM drops below 14V quickly....even to 11V. I have tested 2 different PM, different cables (short and long), I have calibrated voltage and current according to posted instructions... Nothing helps... I am at a loss. just disabling the voltage failsafe, but it doesn't make any sense, I want to fly safely...

    My setup:

    HK X650F frame

    4x AX-2810Q 750KV motors

    QBrain 4in1 25A ESC

    Pixhawk 2.4.6 + M8N GPS, 3DR radio telemetry

    Radiolink AT9+R9D+PRM02 radio system with telemetry

    Turnigy NanoTech 5000 mAh 4S 45-90C battery

    2-axis gimbal + GoPro 4 Hero Black + Boscam FPV transmitter

    Tarot retracts

    Weight with battery is 2400 g (2,4 kg)

    • What props are you running?

      I have an X-666:

      Turnigy Multistar Elite 4006-740Kv motors

      1555 CF props.

      APM 2.6 with GPS

      915mHz telemetry

      Teensy based APM to Frsky Smart Port telemetry

      MavLinkOSD

      200mW Boscam vTx

      CCD camera 

      "Crab" style landing gear.

      2 Multistar 4S 5200mAh LiPos in paralle

      AUW is 2407grams.

      Flight time in excess of 22 minutes (haven't done a full test yet) using ~7336mAh

      3702634680?profile=original

      • I am running 1245 Gemfan props, the frame will not accept bigger props.

        My flight time in hover is 18 - 20 minutes....

    • To calibrate the PM for current:

      1.  Install a freshly charged battery and go fly for 5 minutes or so.

      2.  Immediately unplug the batery.

      3. Charge the battery and note how many mAh's it took to charge.

      4. Down load the last log and look at CurrTot to get the amount of current the PM measured.

      5. Go into parameters and find BATT_AMP_PERVOLT.  By default this is 17 or so.

      6. Do some math:

      Let's say we flew for 5 minutes and the battery took 1500mAh to charge.  When we look at the log CurrTot is 1300.

      Using the formula NEW_AMPS-PERVOLT = (OLD_AMPS_PERVOLT x CHARGED_MAH) / LOGGED_MAH

      We get:

      New = ( 17 (or what ever is in BATT_APMS_PERVOLT) x 1500 ) / 1300

      New = 25,500 / 1300

      New = 19.61

      Plug 19.61 into BATT_AMPS_PERVOLT, click "Write Params", and go fly for 5 minutes and compare the results.   

  • Hampus,

    These numbers are being reported by the power module/pixhawk. I made the mistake of taking them as the truth and blamed the batteries! ;).

    I installed a small watt meter which records the min voltage and max amps during a flight. In my case the min voltage from the watt meter was at least 1.5 V higher than the PM/Pix claimed it was - so no surprise, the Pixhawk triggers a  Battery Voltage Failsafe in only a few minutes when in reality the voltage is still good. I did calibrate the voltage multiplier in Mission Planner.

  • 'm having the two batteries wired in parallel and that should add up to 20C discharge rate=able to deliver 5.2*20=104Amps.

    I have now tested with 2x 5000mah 25C batteries(adds up to 50C discharge rate) and still the copter warns me for low voltage during flight! So irretating!

    --

    Connecting LiPO/Ion batteries in parallel you don't get 2 x 25C = 50C

    since discharge curves of individual batteries don't match in time.

    You need to install Discharge Balancer, alike one built into battery pack coming with robot vaccum cleaner to control discharge current/voltage on each battery individually

  • Can anybody actually confirm that this voltage drop really happened, or is it just a drop measured by the power module ?

    The power module measures the voltage after at least one connector (XT60, EC3 or 5... etc.). Furthermore, we normally use a bit undersized wires in our MR's and there is the reason, where we loose the volts. The situation even gets worse, if somebody connect a wattmeter in front of the PM, as we further increase the resistance of the system and the voltage drop will be even bigger.

    • I am having a similar problem. 

      After a recent crash, I am now getting low voltage (3S < 10.2) battery failsafe conditions after only 2-3 minutes which can be seen in the logs. The real time telemetry confirms a reported low voltage of around 9.5 volts. A few minutes after landing, the volt meter shows at least 1-1.5v higher. Calibrating the voltage in Mission Planner using a voltmeter gives good readings before take-off. 

      To measure the voltage and amps directly during a 8 min test flight (PosHold), I installed a mini G.T. Power watt meter connected in series between the PDB and PM and observed the minimum voltage & maximum amps on landing. The watt meter recorded expected levels, based on the flight times, of around 11 volts min and 16 amps max. I tried a replacement cable between the PM and the Pixhawk power port but that made no difference. So the prime suspect is now the PM.


      Until I get this solved, I have disabled the low voltage failsafe parameter in Mission Planner and now rely on a timer and mAh consumed, which still seems accurate based on comparing CurrTot and the LiPo charger numbers.

    • Sounds a bit like Schrodinger's Cat...

       

  • Moderator

    Hampus, were you ever able to independently determine what your voltage was at hover?

This reply was deleted.

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