Large Voltage Drop After Takeoff

I built and modified a Parallax Elev-8 quadcopter - added an APM 2.6, external GPS/compass, telemetry transceiver, and later a Tarot 2 axis gimbal and 600 mW video transmitter. But I was not happy with the flight endurance time using the stock 1050 kv motors. Wanting to improve my endurance time, I rebuilt the aircraft by replacing the motors with Tarot 4006 620kv motors, 30 A OPTO ESCs, and a UBEC to power the gimbal & vtx. Here's a pic of the finished product:

3691296242?profile=original

The aircraft is powered by a 4s 6000 mAh lipo. Based on a thrust test I saw a guy do with the same motor/propeller combination, I expected flights over over 20 minutes. But a test flight today under almost calm air, produced a flight time, mostly hovering in Loiter mode, of only 3 1/2 minutes before the voltage failsafe kicked in.

I looked at the log file and saw this voltage profile:

3691296363?profile=original

My question is this: is the large drop upon takeoff normal? Perhaps I need to replace the battery? I've had it awhile, using it on my Tarot hexcopter. When I charged it up, the charger read 16.56 V when it said it was full. My multimeter says the battery is at 15 volts now.

Any insights would be appreciated!

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  • Moderator

    @Brian, a couple points:

    - LiPo's are 3.7V nominal and fully charged at 4.2V per cell. I don't know where 4.1V mentioned comes from but unless you're charging in very low temperatures, all LiPo chargers charge to 4.2 per cell by default. 4.2V per cell is the accepted fully charged cell voltage unless you're using something unusual like LiIon or HiVolt LiPo's. (Ref: http://rogershobbycenter.com/lipoguide/, http://www.rchelicopterfun.com/rc-lipo-batteries.html - There are hundreds of guides on the 'net if one needs confirmation or information)

    - So 14.8V is nominal for a 4 cell and 16.8V is fully charged. (16.56V could actually indicate that some cells are not that healthy anymore.)

    - Most LiPo's C-rate is way overstated, 25C packs can often only do 12C, 10C Multistars are only good for 5C or less!.

    If you're experiencing a large voltage drop then the batteries usually can't handle the load or the load is too high due to weight or inefficiencies. Older or budget batteries are bad in this respect. The added weight of a bigger multirotor can quickly cause the current to get to a point that the battery just can't deliver. 0.55V drop per cell is normal under a reasonable load so a 4 cell could go to 14.6V half way through the flight, the fact that yours appears to be reaching that early in the flight indicates that either the battery can't handle the load or the voltage reading is wrong.

    Note: the power modules used by Pixhawk are not precision devices and frequently give erroneous values as has been discussed and reported many times on this forum. Also, the algorithms used to calculate voltage and current are rudimentary, old and haven't been updated or improved on for a few years (not since the first power modules came out).

    Your log starts at ~16.15V seemingly indicating that it's reading about 0.6V under. The way to check is to somehow measure the voltage accurately in flight. If it IS low, you'll need new, better batteries (motors could be over-propped, weight too high).

    If the reported value is low then you'll just need to calibrate or adjust your low voltage failsafe accordingly.

  • The only added weight is the 2 extra motors/arms/props/esc etc but it hovers at a lower rpm due to the extra thrust. I am fine with the extra 5-6amp draw but not sure why the voltage is dropping below 10v, On the 450 I could get 7min out of a 5amp 3s pack. The batteries have less than 10 flights and the IR reading are below 5.  Is there anything else that could contribute to this, or is it most likely the power module? I am ordering another module just to rule that out. I have tried running 2 5amp packs in parrallel just to rule out the c rating but the batteries are rated at 20c.

  • The extra current draw is most likely due to the heavier frame.

    I believe the power module gives inaccurate voltage readings even if it is calibrated in Mission Planner. If the volts are incorrect, then it is very likely the amp readings are incorrect too.

    I have installed a Frsky voltage module (S-port) which gives me totally accurate voltage of all cells which I can monitor in real time on my radio with the XJT module.

    I am now going to install the Mauch power module to replace the cheap PM.
  • I am having a similar issue, with a f450 I was pulling close to 18a at hover and around 1v drop from resting voltage. I upgraded to a f550 and during hover the amps are around 25a and voltage drop is close to 2v on the pixhawk. I hooked a voltage checker on the balance plug and the actual voltage drop is only around 1v. Any idea what would cause this?

  • If you are using a power module, check this. I was getting similar drop off and suspected the batteries. I installed a small GT Power watt meter and the minimum voltage recorded during a flight was much higher than the flight controller claimed!

  • Actually, a full charge on 4S at 4.1V per cell is 16.4V, total.   Based on your log, Voltage looks about right.  As for voltage drop on take-off, you will have a definite drop once the motors spin-up under load.  (My hex running 3805 motors weighing in at 9Lbs would drop to 15.6V under load right after take-off).

    It is possible that you have too much voltage drop in your power leads from the battery to the voltage sensor for the Pixhawk.  Its pretty hard to check that while the craft is flying using a separate voltage meter.  I use a Futaba radio system that has a voltage sensing Rx and that data is sent back to my transmitter.  The Futaba Rx can monitor the voltage at a second point in the system, such as the power distribution board.  This way, one can have a confirmation on the operating voltage.

    When you re-charge you battery, how many mAh did it take to recharge?  Also, over-charging the battery to 16.56V can cause damage internally to the battery, leading to increased internal resistance.  The internal battery resistance will cause your system voltage to drop significantly under heavy load.

    Here's a question, how much current was your copter drawing?  Is the current sensor calibrated so you can check this number?  Here's why, LiPo batteries have a C rating (both for discharge and charging).  If the C-rating is not high enough on your battery, it too could cause system voltage drop under load.  For example, if you find that your amp draw is 30A consistent and you have a 5C rated battery, the 6000mAh battery is only rated for a max draw of 30A (in other words your running at the peak value for the battery).  However, if your 6000mAh battery is a 25C, then it can operate at 150A max (25 x 6).  But under normal loading, it is only running at 5C (or 1/5 the total capability of the battery).  I have found that 25C is about the most optimal rated battery for most 450-550 class multi-rotors.

    Lastly, you could just have a battery that is going bad...

    • Joe,

      Thanks for the info and questions. Perhaps I did overcharge the batteries - I am using a Tenergy charger set at 14.8 V (4S), 4.8 A - it charges to, I believe 16.6V before indicating that the battery is full.

      I don't know what the current draw is as I don't have a current meter to calibrate the current sensor on my APM controller. The battery label indicates 25-50C Discharge.

      I think I will invest in a current meter and buy a couple of new 4S batteries - one for my hex and the other for the quad.

      Thank you for the info - it has been helpful!

  • A chaged 4s should be 14.8v of not check charger calibration, or your multimeter.
    Then, a calibrated power module (using a good multi meter) should display correct voltage too, once you have trustworthy charging and metering in place, post such log, with current. (Preferably calibrated too, but that's not anyone can do well)
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