View failsafe cause in Mission Planner (APM 2.6)

I need to see what caused my failsafe in Mission Planner DataFlash logs because my APM throws failsafes generally around 11.4 volts. I think it’s a brownout of some kind (why else would it be voltage-related?), but I don’t know what is browning out, receiver, GPS, APM, or some combination of the three? When I auto-analyze the log it says Brownout = PASS but GPS = FAIL, so I’m guessing the GPS is bad, but I want to make sure by looking at the failsafe cause in the logs.

Could you possibly attach the dataflash log onto this thread so I can have a look to try help.

Oh wait, nevermind. I found out how to do this. Apparently the offending part is my battery monitor (or possibly battery) but sometimes (and this is the weird thing) the GPS causes the failsafes too! If I wait for a good 3d fix it’s usually the battery monitor though. It thinks the battery voltage is way lower than it is. Is it time to get a new power module? I will send you some .bin files when I have the time (it’s 8:00 right now)

Actually, I’ll just tell you what happened: It all looks ok (with some errors followed by ECode 0 because they get cancelled) but then at the end the battery monitor glitches and FS_BATT failsafe triggers because the voltage is “too low” when it really isn’t. Do I need to redo that power module calibration?

Happy to hear you figured it out. And i would try to reconfigure the second thing could be a battery issue, what is the discharge rate of your battery and what ESC are you using? As it could be that your battery cant provide enough current to drive your multirotor, causing the voltage to drop when you apply thrust.

That’s a good suggestion, but it would be the current dropping not the voltage right? Anyway, here’s my power setup:
Battery: Zippy flightmax 3000mah 20c (30c burst)
ESC (slightly overpowered): Afro 30a with 5v 0.5a linear BEC
Motor: Emax mt2213

So hopefully my battery can support 3ah * 20c which is 60 amps. I’m not sure how many amps are drawn by the motors but I think I might be cutting it close. I will find out how many amps those motors draw at max. throttle…

A couple of questions :slight_smile:

How old (how many cycles) is your battery?

What props are you using?

What is the total rtf weight of your drone?

Is the BEC the only power source for the avionics?

How many ESCs have their BEC connected?


Thanks for responding,

My battery is pretty new but I did accidentally short-circuit it once (it seems to work fine, amazingly).
I’m using 10x4.5 props.
The total weight is about 1.2kg.
No, the pixhawk power module (rated for 90A) supplies power to my ArduPilot Mega 2.6.
All four ESCs have their BEC connected.
Does that setup look OK?


First I’m making an assumption that you’re using a 3S battery? I hope so anyway :slight_smile:

Can you try another battery? (there’s a small but unlikely chance you’ve damaged it when it shorted)
Using that motor/prop combo should be drawing around 12-14 amps in the hover, so well within the capability of that size battery.

Just had a look at the Pixhawk Power Module specs and it’s supplying 5.3V at upto 2.25A, so that should be okay too as I can’t see your avionics drawing that much current.

I’d disconnect all the BEC power lines from the connectors at the ‘servo plug’ end. It was always a recommendation from ESC manufacturers that only one BEC should be connected to supply power as two or more connected BECs would fight each other, but as you’re using a separate 5V supply, there’s no need for any to be connected and just run the ESCs in ‘OPTO’ mode.

I’ve been reading up a bit more on APM, and there are so many ways that it can enter failsafe, I’ll finish reading and get back to you.


Thanks for responding,
Yes it’s a 3S battery. I would try another battery but I don’t have any others (unless I solder three 1S LiPos in series, but that would be difficult and I didn’t take good care of those super well anyway as I over-discharged them a lot…)
I’ll try disconnecting the ESC servo plugs. Hopefully I’ll be able to put them back in if need be. Or, if I can disconnect positive and negative (will that work if the signal cable is the only one connected?) I can just flip the wire around and connect the signal cables but leave the positive and negative hanging off the other end. That would only work if the plastic casing weren’t there though because I think it prevents that :frowning:. Oh well, I’ll just slide the red plug out on all the ESCs like you said and hope that I can slide it back in. I’ll see if that fixes the possible power issues although I don’t think it explains the power module thinking the battery voltage is dropping…

Yes, just remove the red wire from the connector as you need the signal and ground to remain connected. There’s a little plastic tab on the back of the connector that you lift slightly and the cable and pin should then just slide out.

I’ve been reading up on failsafe and power module settings, (a lot more to read through though :frowning: )

It looks like there could be a problem with your failsafe settings. As a 3S battery has a nominal voltage of 11.1V it shouldn’t be entering failsafe at 11.4V which seems much too high, (this could explain your why you’re getting a ‘low voltage’ failure).

The APM will enter (voltage) failsafe when either of the following conditions are met;

  1. Voltage drop below LOW_VOLT for more than 10 seconds.
  2. Your flight uses more than 80% of its capacity.

What voltage have you set your LOW_VOLT to?
and what have you set your BATT_CAPACITY to?

I’d recommend 10.5 for the low_voltage_cut_off, and although I’m tending toward the voltage setting, it won’t hurt to check that the BATT_CAPACITY is set to 3000 (mAh)


I removed the BEC from the ESC, but it didn’t fix the problem. LOW_VOLT was already at 10.5, and I set BATT_CAPACITY to 3000mAh with failsafe at 600mAh (20%). Any other ideas?


Which brings us back to possibly a faulty battery, and trying a different battery, ideally the same, but a higher C rating will confirm if the problem is with the battery or not. Can you borrow one to test with?

What actually happens when a FAILSAFE occurs?


It returns to home, which is what I set it to do. I could turn that feature off but then when it gets to actual low voltage the LiPo would get damaged and the ESCs would cut out. Last time that happened (with my old KK2 board) two of the ESCs cut out before the other two causing it to go careening to the left and smashing into a house (luckily it only broke an arm).

I don’t have other batteries to test with, except a few 1S LiPos. I could use those in series except they’re each like 20C 500mAh so I definitely won’t get to draw the amount of amps I need :frowning2:. Generally, when I graph CURR.ThrOut and CURR.Volt in Mission Planner, as ThrOut goes up Volt goes down almost by the exact amount ThrOut went up. Does that mean anything?


Yeah that does sound like the battery is the cause :frowning: have you verified this with voltmeter just incase power module is misbehaving?

Also I am assuming you calibrated your power module?

I did try to calibrate my power module, not sure if I did it correctly. I don’t have a voltmeter, I’ll have to ask a friend who has one. Hopefully this problem can be resolved by Halloween so I can have a good amount of flight time with a costume on my quadcopter :slight_smile:.

Yes, you need to calibrate the power module for VOLTAGE and CURRENT. The procedure out lined here: Power Module Configuration requires using a watt meter to calibrate current, but I have a much safer and more accurate way…

The basic process is you take a fresh battery and make a flight of 5 minutes or so, land, disarm and unplug the battery. Charge the battery and record the mAh charged. We will call this number “CHARGED_MAH”

Now take a look at the flight logs and find the total current used. We will call this number “LOGGED_MAH”.

Next look at the BATT_AMP_PERVOLT parameter and record that number (by default its something like 17…). We will call this number “OLD_AMP_PERVOLT”

So, let’s say LOGGED_MAH was 1300Mah, CHARGED_MAH was 1500mAh, and OLD_AMP_PERVOLT was 17.

We can use this formula:


NEW_AMP_VOLT = (17 x 1500) / 1300
NEW_AMP_VOLT = 25,500 / 1300
NEW_AMP_VOLT = 19.61

Set BAT_AMP_VOLT to 19.61 and click “Write Params”

Take a fresh battery and make another 5 minute flight.

After one or two iterations you should be well within ± 200mAh…

For the low voltage warning, set it to something just above 3.2 volts x the number of cells and set the reserve current to 20% of the battery capacity. For a 3S 3000mAH battery this would be 9.6V and 600mAh respectively.

The reason for using these numbers is you want the low battery fail safe to always trigger on low capacity and never on low voltage.

I do this with my APM quad and my Pixhawk hex, and the end of flight at rest cell voltages are never below 3.7 volts.

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Ok, I will follow those steps ASAP (even though I’m now rather sure that the battery is the problem not the battery monitor because when I plug the battery in without running the motors the voltage is always correct but when I apply throttle voltage drops) and hopefully it’ll help.


You said to record the charged mAh. How do I measure that?

In the display of battery charger