Battery Issue???

I have a 2250mAh 11.1v 3 cell battery that I'm using for my 450 kit build, motors are 1000Kv motors. 

Using a Pixhawk with the PX4v1.3.3 flight stack, loaded via QGC.

And I'm using an Imax B6 charger for the battery. Battery is brand new.

This is the issue; I will plug the battery in, with the props off, as a system check. I can throttle it up all the way, no problem. QGC says batteries are at 100%. External voltage meter says all 12.1, no1 4.03, no2 4.03, no3 4.06. 

System check looks good, so I put the props on.

I get the UAV up and within seconds, QGC tells me that the battery has dropped to 33% and the Pixhawk battery alarms go off. The external voltage meter, that is set at 3.6, does not go off. 

I keep it up for a few more seconds and the battery level drops to 17% and the alarms go nuts. I set the UAV down, for a flight time of mere seconds. 

I will then unplug the UAV from the battery, wait a second, plug it back in and reboot. QGC will say the battery is still at 100%. External voltage meter hasn't changed. 

So with the props off I have no issue, but as soon as I put the props on, the battery can't seem to handle it.

What am I doing wrong?

Thanks you for any insight.

Cheers!

_M

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Replies

        • I have a heavy drone (I should have listened to the good advice when I was planning my build)

          As such I need big batteries.  My battery education has been expensive.  One of your priority's needs to be preserving your battery.  Its very simple to damage the battery, reducing its capacity, and you may not even know it.

          A good article on understanding batteries :  http://www.rchelicopterfun.com/rc-lipo-batteries.html

          skip down to "Lipo battery ratings."

          First never discharge more than 80%.  Second, don't allow the current to come close to the C rate.

          Most think that as long as the voltage is over 3v/cell everything is ok, but that's not the case.  The rule is UNDER FULL LOAD the voltage can never drop to 3v/cell.  

          Depending on your setup, during flight and under load the voltage drops, sometimes as much as 1.5 volts, after you land and you have no load on the battery, the voltage comes back up, making you think the battery was not used much at all. 

          The C rating is the maximum discharge current.  The closer you are to the C rating, the more the voltage lag will be.  As Alasdair stated the C rating is not to be trusted.  I now use LIPOs with a high C rating so I will never exceed half of the stated C rating.  That seems to work for me now with lass than 0.5v lag most of the time with a calm flight.

          Mark,

          As for how to proceed, I would get a battery monitor that will plug in between the battery and the drone during flight and will read max / min voltage, AND current.  I put a link in the previous post for an inexpensive unit.  Watch the min voltage during the flight.  Watch the max current during the flight.  Record you information, this info holds the answer as to whether your problem is just the settings in your ground control / flight controller, OR, you just need a better battery.

          I suspect your in for a new battery,  In this case, if you can handle more weight, you can max out the mah, its possible to go to 5000mah, with a C rating double your current battery.  (your pic doesn't show a C rating on the current battery).  Then monitor your battery, with resolve, to avoid damage.

          I have personally destroyed seven large lipos over the past few years, simply by discharging to much capacity, partially due to the large voltage lag caused by drawing more current than the battery was able to handle.  These LIPO batteries are delicate, and the rules are rigid, and the consequences are expensive,  good monitoring, and constant analysis, are essential.  Please keep us informed of your progress.

  • I use mission planner rather than QGC.

    From my perspective your battery settings are not right.

    I would expect to see the following

    max cell voltage: 4.2

    min cell voltage: 3.0

    max discharge: 80%, after testing and verification you might go to 85% discharge.

    I see your ground control software assumes 5C (current draw)  I dont know your UAV, but it is possible that its pulling double that current.  this could be causing a larger than anticipated voltage lag during flight, (under heavier current draw) that would cause your voltage reading to drop during the flight, trigger the failsafe, and after you land, the voltage, with no load, reads fine.

    As you raise your throttle, and current increases, the "voltage under load" drops.  This lower voltage is what triggers the failsafe.  When the current is reduced or cut off, the voltage rises again.  

    Regardless,

    You need an independent power analyzer that you can fly with.  I use this:  http://www.ebay.com/itm/like/400751294822

    This is a small, not an expensive unit, after each flight I copy down the reported peak current, max and min voltage during the flight, and the mah used.  Only after an independent reading this can you confirm if you settings are correct.  Your volt meter will not tell you the minimum voltage encountered during the flight, and that's the issue of the moment.  

  • I haven't used the QGC yet, only the Mission Planner, so I am not very familiar with it. But there has to be a setting in the QGC, where you can adjust the battery values or setting in general, have you had a look at that?

    • Francisco,

      Yes I have. I posted those settings two post ago. Here are those screen shots in the body of this message, so you don't have to click. Again, as far as I know (not much) these settings are correct. Am I wrong?

      3702637589?profile=original

      3702637526?profile=original

      3702637397?profile=original

      • This screen is missing two key parameters which are used to adjust things: 

        BAT_V_DIV - Voltage divider you can adjust if voltage isn't reading correctly

        BAT_A_PER_V - Amps per volt value you adjust if current isn't reading correctly

        Both of these are currently available using the parameter editor.

        Latest version of QGC (not yet available as daily build, hopefully by tomorrow) has these parameters exposed in the Power screen:

        3702747884?profile=original

        And there are calculators to adjust then. Here is the on for voltage:

        3702747909?profile=original

        • Oh, alright!

          Can I use my external, on-board voltmeter, seen in the pix of my battery, to measure voltage for the calculator? If so, I'm all set. I just need the new daily build, is that correct?

          Cheers!

          _M

          • "Can I use my external, on-board voltmeter, seen in the pix of my battery, to measure voltage for the calculator?"

            I would think so. You then type that value into the calculator, Hit Calculate and it will set the new value of the BAT_V_DIV parameter for you. You should then see the voltage in the dialog change after you hit calculate to be closer to what you external meter is measuring.

            • Don,

              I suppose I should drain the battery to around 3.4v per cell, give it a full charge then read and calculate. That's what I'm going to do, once the new QGC build is ready. 

              I'll let everyone how it goes.

              Cheers!

              _M

              • New Daily builds are posted: http://www.qgroundcontrol.org/downloads. Note that you need to run PX4 firmware master build as well since it has changes for those parameters. Stable PX4 firmware has them set to -1 which doesn't allow you to recalc them.

  • Mark,

    Your battery charger should display the mAH added to the battery during charging.  If you keep a log of the energy added to the battery after each use then you will have an independent assessment of the energy used in each flight.  You can use this to help calibrate the PM.  The objective is to avoid discharging more than 80%, based on the mAH use, not necessarily voltage. 

    Have you done an analysis of your system with Ecalc?  (Ecalc.ch)  Your battery might just be to small a capacity or not have a sufficient C rating.  The fact that you lost a cell suggests that you may have discharged over 80%, or the current draw (C rating) is more than the battery is rated for, causing damage that would not be apparent, but cause a significant permanent drop in capacity.

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