Battery Tests

Discussion in 'ALTA' started by JoshuaDunn, Sep 4, 2015.

  1. JoshuaDunn

    JoshuaDunn New Member

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    Hello Alta Folk .
    Has anyone else tried using one large battery pack on the Alta yet , I tried out one yesterday , It was 22000mah 25-50c tatuu weighing in at 2509 grams . So I saved 300 grams on the usual 10k mah I have been flying with . But now I am not so sure it was a good idea buying a few large batteries . I was flying a C300 on an M10 with a canon lens , all up my pay load was only 4.6kg plus the 2.5 kg battery , it flew fine untill I gave it a serve and ramped up the power to do a fast rise , the red low voltage light went crazy ........ So Im wondering if it is even safe to fly with a single pack . Has anyone else tried this ? I switched back to my 10 K mah dual set up and repeated the move , no red lights . It says in the manual one pack is fine with the correct discharge rate which I thought I had .
    Any Ideas ? Cheers , Josh
     
  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Josh:
    What you're seeing is living proof of Ohm's Law. The amount of current a battery provides can cause the voltage to sag based on the internal resistance of that battery. The internal resistance can be thought of as an external resistance, and the current flowing through it causes a voltage drop across that resistance -- and that's what you're seeing.

    The problem with a single battery pack is that the ALTA can pull a lot of current out of the battery -- and the internal resistance of the battery then limits the amount of current that it can provide and causes the voltage to sag.

    With dual batteries, the current from each battery is halved and, because the batteries are connected in parallel, the internal resistance of the two battery combo is also halved -- this means the current can be provided without as much voltage sag across the internal resistance of the batteries.

    So a single pack *is* fine, but within the limits that Ohm's Law and the battery's internal resistance and the maximum discharge current rating allows for.

    You probably need to use a battery with a higher discharge rating, e.g. 100C. It might also be that the Tattu really doesn't provide 50C discharge. I've not testing that particular battery.

    Andy.
     
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  3. Dylan Glockler

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    One of the test pilots at Freefly said he melted a battery connector flying one battery's during testing and they said absolutely do not risk it with any load and wouldn't recommend it at all in fact. Don't mess with ohms as Andy indicated.
     
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  4. JoshuaDunn

    JoshuaDunn New Member

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    Thanks guys , I guess maybe they should think about taking the single pack option out of the manual so sucker fools like me don't go trying it with a 5 kg load on the drone . Melting cables sound like a bunch of fun! Said no one ever!
     
  5. Ozkan Erden

    Ozkan Erden Distributor

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    25C discharge rate on a 22A battery means, it can provide 550 amps continuously. But in practice, I doubt this.

    I have very bad experince with Tattu 22A batteries. I had 6 of them and all of them went puffy after 20 cycles. 2 of them became unusable since one of the cells went down. My average current draw was around 120 amps. I was using it as a single battery.
     
  6. Steve Maller

    Steve Maller UAV Grief Counselor

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    Ouch! That's an expensive experiment. I hope you were able to get Tattu and/or your distributor to replace them or otherwise take care of you.
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    It's quite sobering to realize just how much electrical power we're dealing with on this class of copters. I've seen sustained currents, as Ozkan says, of 120 Amps - 150 Amps. If you consider worst case (which is 4S -- the lower voltage means you need more current to do the same amount of work), that's 2,400 Watts.

    A typical electric kettle (which we Brits use in times of crisis to make a cuppa char) typically pulls 2,000 watts.

    Add to which, that the heating effect of electrical current is proportional to the square of the current (P = I x I x R -- can't do a superscript!) So, if you've got any significant resistance in the EC5 connector, it becomes a remarkably good soldering iron...well, de-soldering iron.

    That's my personal reasoning behind the idea that 6S is better than 4S -- the increased voltage means the batteries can deliver the same amount of power at half the current and a quarter of the heating effect. But, hey, I'm only a porgramer pergrummer purgrommer a guy who writes compewter code so someone with an EE degree doubtless can 'splain it better. :rolleyes:

    Andy.
     
  8. Gary Haynes

    Gary Haynes Administrator
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    Many of the battery manufacturers are notoriously inaccurate on their C discharge ratings. Accuracy similar to "I'm from the government and I am here to help". And as previously mentioned while the battery might handle it the connectors would need to be very robust so that they could handle the total amps required. Good video from Progressive RC.



    EC5 connectors, pretty common are 120amp connectors. You didn't mention the connectors on your rig. Largest PRC6 is a 200amp connector.
     
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  9. JoshuaDunn

    JoshuaDunn New Member

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    Hello Fellow free flyers , I have another battery question for the electrically inclined to ponder .I have recently looked into some smaller batteries so I can safely fly at higher altitudes with our current set up . I found some 6200mah 6s 25-50c LPB power batts in NZ as I cannot get anything freighted to me from any other countries due to the new flying rules . Im just checking if two of those batts will supply enough amps for the craft , I did the equations and they state single pac must have between 200-400 A , do they mean the combined power of two? which for my 62s is 310-620 A or do they mean each single pac much pack that much punch which would limit me to flying 8k mah only at 25-50 c . The single pack is 300-600 A which as you know I tried and failed with the big boy 22k mah 25-50c . Its like learning a new language this battery thing . Cheers , Josh
     
  10. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Check one of the flight logs that the ALTA records (the microSD card is under the little black cover on the GPS unit).
    The log file is a text based comma separated variable file that Microsoft Excel can open.
    Column CM to CR contain the current being drawn by the motors, and CU to CZ show the wattage.
    (Not quite as relevant for your immediate question are columns DC to DH which contain the Watt/hours drawn)

    The good news is:
    1. Obviously the copter doesn't know how many LiPos are connected, it's just wanting to inhale electrons.
    2. Two batteries in parallel will double the batteries combined ability to supply current.
    3. Dual batteries are much kinder on the battery (less chemical stress during the discharge).

    You can use the data from CM to CR to calculate the total peak current draw flying your ALTA configuration. Then you will know just how much current the batteries have to deliver at peak demand -- you might want to add a bit extra on top, say, another 10 - 20% to be safe.

    The quickest way to do this is a bit counter intuitive:
    1. Select the whole spreadsheet by clicking in the square above the row number 1 and to the left of the column letter A.
    2. Then select Data in the main menu, then Sort, and enter CM in the Column, and select Order of Largest to Smallest.

    This will then sort the entire spreadsheet so that you can immediately see the maximum value in Column CM. Make a note of the value now in row 1 of column CM.

    Repeat this Sort for each column from CN to CR and you've got the maxima for each motor. Add them up. That's the maximum current draw during the flight.

    In my case I got 347 Amps -- I was flying with just the lower battery tray "landing gear."

    Then it's down to determining whether the LiPo's that you want to use can deliver that amount of power. That's where the 25-60C number comes in. The 25C means that it can deliver 25 x 6,200 milliAmps -- which is 155 Amps. (The 60C is the maximum burst rate not maximum continuous).

    So now you know that if you put two of these in parallel, then they can deliver 2 x 155 Amps, which is 310 Amps.

    This tells you that you're pushing it and probably should not use those LiPos.

    Andy.
     
  11. JoshuaDunn

    JoshuaDunn New Member

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    Thanks andy , a great detailed explanation . I dont have a PC so I wont be doing any exell spread sheets .
    So in normal people speak , you are drawing 347A at your peak in flight , and my light weight lipos running together give me a peak of 620A and a steady 310A at a minimum , so surely thats plenty !
    When you say you only had the battery tray, that was a completley empty craft? or with a load ?
    And if your drawing 347 Why would free fly put it in its manual that a single battery can have a lower draw of 300A? Very confusing .
    Plus I also checked up on my EC5 connectors , they are only rated for 100 to 120 amp draw , so how come they are not melting .
    Its like a bunch of crazy wizards got togther and invented lipo batteries just to mess with us muggles .
    Cheers , Josh
     
  12. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    You can use Google Sheets -- it's Excel compatible, and available if you have a Web browser.

    I re-read your post. Can you tell me how you got to 620 Amps, please?

    Empty ALTA.

    I think they're just quoting the minimum -- I was more try to illustrate how you could do the calculation.

    Nah. It's all about Ohm's and Joule's Law of Heating.
    Andy.
     
  13. JoshuaDunn

    JoshuaDunn New Member

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    Hi Andy , I got the card out , it opens right up in Numbers on my Mac , what a lot of numbers , so much info , by the second , mind blowing really .
    I got 620 A as my peak output from my 2 6200mah 6s 25-50c lipos , 310A being the continous . 6.2 x 50 doubled
    I will try and calculate my peak with my max payload using your method
    thanks again
    Josh
     
  14. JoshuaDunn

    JoshuaDunn New Member

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    Hey Andy , I had a play around with graphs , My average draw per motor is 20 amps , it peaks at 31 on one motor for a few seconds but mainly I am in the low 20,s . Thats me flying at 1000 feet AMSL with max paylaod , so my average draw is 120A , peaking at 180 A maxed out , not sure how you arrived at 347 , but that seems like a lot with an empty craft . But I could be adding things up wrong as I am not a super mega tech head like you . Food for thought though . Cheers , Josh
     
  15. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    In all probability it was because I was deliberately "pushing the envelope" and testing how fast the ALTA would climb etc. I would tend to trust the data that you got -- but why don't you try and upload your .csv file and let me double check? I *think* the forum software allows for .csv files to be uploaded, but if it doesn't just append .txt to the end of the file name -- that usually fools it. If it turns out to be too big, then create a Zip file of it -- that *certainly* fools it.

    Andy.
     
  16. JoshuaDunn

    JoshuaDunn New Member

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    Good morning Andy , it is for me anyway in NZ. I had a bit more time playing around with the graphs and charts . What a compelling list of numbers it has become , Its like looking into the matrix for the first time .
    I have found some impressive spikes of amp draw when the craft must be making some large wind gust correction or some such event , there was a 55A spike on one motor and a 48A spike on another , but never across the range , so as loath as I am I have to question your methodology at getting what you called the maximum current draw during the flight , by taking each individual motors top draw across the whole flight and adding them togther into a moment of time that never existed seems like a very inaccurate way of estimating maximum draw for the motors as a collective during flight , when one or two spike the others always seem to be lower because of the physics involved during the moment that the correction is made . I appreciate your help with this matter for sure , but if we want to get the true maximum draw we would have to use a differenbt equation to reach a conclusive figure . One that escapes my grasp at this time , but who knows , I may crack the code in time using the law of averages and good strong brick wall on which to strike my head against .

    Many Kind regards , Josh
     
  17. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Don't hesitate to question my methodology, Josh. I was just trying to show you the principle involved.
    If all you're seeing is a momentary spike, then it certainly will not cause a problem with the connectors.
    However, if the LiPo's internal resistance is too high, then you may find that the Lipo will simply not allow the spike to occur -- and that will manifest in less stable flight because the flight controller will not be able to correct as quickly for a wind gust, say.

    If you want, you can use Google Sheets to calculate something less than the peak current -- it would be your choice of formula.

    Andy.
     
  18. Adam Orens

    Adam Orens Member

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    :eek:Was the 347 amps at full throttle?
     
  19. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I'll have to double check....I wouldn't be surprised as I did do a couple of "very surprised cat" ascents.

    Andy.
     
  20. alex ryan

    alex ryan Member

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    This might be a stupid question, but how do you get the card out of the GPS unit? Do I need to use tweezers or is there a simpler way? It doesn't seem to have a push in, pop out release mechanism.
     

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