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

    • Yes, It was 14,8V during hover with fully charged batteries(16,4V).

      I'm not arguing with Clifton about my hexa being underpowered to pull 5KG and I do really appreciate his help. In fact, I have now completely taken the hexa apart and I'm now selling the power system to upgrade to 6S.

      But! What I am saying, is that both Hobbyking Power Module and the Attopilot 180A has a weak design because they are not reliable in any way when measuring voltage during high load. My ampmeter tho, is. But I can't carry that on the hexacopter during flight now can I.

      • Two Multistar 10A 10c batteries here among many others. As far as I'm concerned their true C rating is between 2-3 (3 is even pushing it). They may seem ok new, but degrade rapidly and voltage drop is huge. IMO if your copter is >2.5kg the MS is about useless if it is expected to extract 80% of capacity. It isn't going to happen,and by chance if it does, even a blind squirrel finds a nut occasionally.

        I use a Frsky Lipo sensor and use that to compare to the PM while flying, then a good quality meter on the ground after landing to verify the whole package and tweak the PM in at the lower end of the voltage as they aren't real accurate linearly (run cells down to 3.6) which is where my concern is for knowing when to land.  I think it is wise to know the voltage of each cell anyway. Using a Teensy with the radio you can trigger an alarm for individual cells. It seems that larger AUW copters that gobble up lots of current need something better than the common PM; a high quality hall effect type seems the way to go.  

        Have since switched to Lumenier and Tattu batteries.

        It does look like your motors are underpowered as you stated. 

        What receiver are you using?

        EDIT: Big discussion on MS here. I offered some test results, but many ignore the results. They'll have to learn for themselves.  http://www.rcgroups.com/forums/showthread.php?t=2207137&page=56

        • Wow, I guess I'm a blind squirrel then.

          I tried to wade through that thread, but these old eyes got tired. Can you point me to a particular post in that thread?

          FWIW, I'm running MS on a light 500 size quad and a Tarot 680 Pro.  The Pro tips the scales at 3257 grams.  Flying 2 4S 5200mAh in parallel I get consistant 18 minute flights. Hover current averages 18 Amps as measured by a clone PM that was calibrated "live" (motors running with props attached) using a GT Power RC 130A Power Analyzer. Total current used and current replaced are consistently within +-200mAh.  Battery charger is a Turnigy Accucell 6 80W/10A powered by an HK 350W/25A power supply.

          I have RTL set to trigger at 20% capacity remaining and/or a battery voltage of 12.8V.  To date, RTL has never triggered on low voltage, and individual cell rest voltage 5 min after a flight is consistently above 3.7V. 

          3702802206?profile=original

          • I can't find the post either, I truly remember me writing about it in this thread. I must have never pressed to post button. 

            What is an important part of the story is that I got to borrow my friend new 4S 65C batteries, and the voltage drop on the HK PM was still as big.

        • Please remember that I got as big of a voltage drop even when I used 65C batteries, so I don't think that it is the multistars batteries. I have ordered 6S multistar batteries for my new 6S setup(810mm arms, Tarot 4114, Xrotor 40A). Should I not have done that?

          I use Turnigy 9X receiver.

  • another thing dont rely on the onboard power management its faulty on the pixhawk its only 60% accurate at best 

    the voltage and amp senseor that you plug into with your pack is cheaply made in china i once saw them in a secretly recorded youtube video 3dr cheaply makes them in china and there are knock offs too...

    anyway dont trust your output when it says youve used this... go based off time... do a run at half throttle then land and test with a meter.. do this till you get to 3.8 3.5  somwhere in there... then record how long that tooke at  hovering and at flying around and flying around fast racing.. 

    then average the time take the 3 times and average them out.. then stick that time on a timer on your watch or remote or what ever and go by that with it goes beep land that simple..

    the sample rates of the pixhawk voltage and these beepers is slow at best  i calculated once 10 seconds slow in reading when you go from flying around to a hover.. and remove the stinking failsafe on your copter just disable it will falsly go off at 4 volts there un reliable..... just go off time its the only proven method.. if your bat is max 6 minutes before dead dont fly to max fly to 5 minutes to give you time to come home and land..

    its common sense guys

    that and use new batts new connectors rubber band them or tape them if you have to etc..... ive been flying for 20 years and seen some people push things... just drive it like a grandpa on a sunday after noon and your stuff will ast

    • I beg to differ yet again.

      I have an APM and a Pixhawk, both running your so called cheap Chinese knock off battery monitors.

      Admittedly out of the box they are not accurate, but if one does his due diligence and actually calibrates them for voltage and current, it is possible to have measured voltage with +- .02 volts and total current used within +- 200mAh.

      And just so you know, I've been at this for over 25 years.

      • cliff when you been flying as long as i have you dont trust electronics... if its made by man it will fail... eventually 

        use the timer method  its far more acurate than a onboard  current sensor..

        china makes crap things and there are some things that are made great.. its a crap shoot... 

         but the timer method if you do the basic calculations i stated.. take 3 readings idle hover and hard flight and average them youll get your buffer zone of time... then take off 1 minute depending on how far you are out.. and common sense...

        • CHINA DOES NOT ! make crap things. that's an old notion just like Germany has "best" engineering.

          China will be the next japan in 10 years. Japan was in the same crosshairs in the 70s. Very soon we will have only the best made in China. There is a reason why so much is made in China. China is a very experienced country in manufacturing. Top machinery and tools are made in China with great quality. So that statement was true in the 90s. Not 2015.

          Ranting but people don't just say stuff. Have you been to China have you seen how they manufacture ??

          • Moderator

            Tony 

            Wyatt is correct and I am also a long standing Automation engineer, So I understand this problem very well.

            China DOES have upto date production facilities and good workers but they dont waste valuable time on Quality control, its still a crap shoot to see what you get, ask anyone who has had an issue with hobbyking products and tried to get a refund.

            Yes, I've been to and worked in China and my view is the same, some great, some crap.

            In this case the overloaded copter will never fly correctly and needs a diet badly. you cannot ignore gravity. its been around for a long time.

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