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.



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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?



"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.


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.



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.

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.  


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'm not sure if this is the problem, but there's a significant voltage drop on Lipo's when a load is applied. The current drawn while spinning the motors without props is tiny, but with props on the current drawn by brushless motors can be very high.

For example my Hexacopter shows 12.6V sitting on the ground with a fully-charged battery, but as soon as I take off the voltage immediately drops to about 11V and then slowly drops to 10.1V (about 3.4v per cell) when my battery alarm goes off. At hover my copter pulls about 30 Amps on a 3S Battery and I get that large of a drop even with a 5800mAh 35C battery. After the alarm sounds and I land and stop the motors, the resting voltage bounces back to about 11.5V.

Can you try to fly with the voltage meter/alarm attached and see if the low voltage alarm is triggered on that? That should tell you whether the voltage on the battery is actually dropping under the load of the motors or if it's just a calibration issue within QGC that's prematurely setting off the alarm.

Sorry I can't help with QGC as I use Mission Planner for my builds. 

Harry & John,

Your analysis seems the most plausible. I can see how "voltage drop" could be the issue. I'm going to look into this, rather than relying on a new build for QGC. Thanks again for your informative replies.



Another thing to keep in mind is the 'B.S' C ratings that are printed on most lipo batteries which are to be taken with a grain of salt. We have tested countless makers batteries and none can deliver their claimed C output for more than a few seconds, and even then, only when the pack is at 60 degrees optimum temperature. The rating you are looking for does not exist in published data from manufacturers, and that is the continuous C from fully charged to safely empty. The closest you will find is manufacturers claimed continuous ratings, fudged like 1980's ghettoblaster wattages.. {like 2000watts output.. from alakaline batteries! lol} Ratings of 25C continuous 45C max!.. yeah right, if 10 seconds is what you class as continuous. The best we came up with was 8C, when rated to 45 continuous 90 burst, and even then the total energy yield suffered to below 70% of pack capacity, most 30-45C lipo's will usually safely get 4-6C with around 75-80% yield after a dozen cycles, and again, it requires everything to be right. 6C equates to ten minutes flight time, pretty woeful, but then some multirotors are damned heavy so that's what is needed. Pushing beyond 8C on 35-45C packs guaranteed premature failure of every type we tested to under 20 cycles. Just something worth factoring in when calculating battery payloads, divide the label value by at least 6.

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.


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.

Bringing this back up.  I have the latest QGroundcontrol with the new battery calculator on the Power page.

I also have a Power Meter inline on the drone.  Here are the numbers I registered:

Battery:  5200mAh, 3S, 11.1V, 15C

454.4 Wp (

28.2 Wh

2.542 Ah (Capacity Drain)

40.32 Ap (Max Current Draw)

11.01 Vm (Minimum Voltage)

Under stock settings, the alarm went off when there was 52% battery left (measured on charger), measuring 3.82V per cell.  So with 2.542Ah drained out of a 5.200Ah battery, that is correct.

So if the alarm is going off, I have some parameter wrong.  So if I use the new Power Voltage and Amps per Volt calculators what values do I use based on my power meeting readings?

For Voltage Divider: Do I put in Vm, which is 11.01 Volts?

For Amps per Volt: Hmm, what do I use, Max Current Draw, which is 40.32 A in this case?

I am trying to find the forum link that explains this calculator, but haven't found it yet.  If anyone has any insight, let me know, thanks!




I rebuilt my quad. Some of the solders on the PDB were "weak". I replaced the PDB and upgraded to new 4cell. Happy to say that the power alarm issue is gone.

But now I have a wild counter clockwise spin issue, that I didn't have before. It spins really fast, so fast that trims will not counter. It didn't spin before when I had it up for seconds.

So... one problem solved and another one started. 

To all those people who bought DJI's and flew them out of the box... I hate you. 

A motor is spinning the wrong way, a prop is wrong, etc. When it spins out of control its because the computer is trying to stop it but the backwards motor or prop is doing the opposite effect.

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