Lowbattery failsafe and crash

Hi, I have my hexa, with a pixhawk 1 and copter 3.5.7 on a 4s 5000mAh battery. I have configured a 13.8V failsafe. I have a few flight of over 9 minutes after failsafe trigger and my hexa landing without a problem, always in POSHOLD flightmode. In the last flight I was testing some script in GUIDED mode and after 7.5 minutes or so I heard low battery tone and a couple of second later it fall to the ground and crash. Analizing the log file, it seems that failsafe was triggered with 12.8Volts 12 second later of reach 13.8Volts limits.
Could someone confirm my analisis and try to explain why?
Is 13.8V a too low value for a 4s battery?



The google drive link is not public so I don’t have permission. But anyway, I can tell you that based on your description:

  • 13.8v is definitely a very low setting for a 4S 5000mah battery. That’s giving you very little time before it no longer has enough juice to maintain altitude or arrest a descent. I suggest 14.0v.

  • Note: It is also not a good practice to discharge these batteries below 85-90%. So cutting your failsafe short usually results in 90-100% (or more with a crash) battery utilization, which shortens their life dramatically.

  • The failsafe activates once the voltage has been below the setting for 10 seconds. So once you dropped below 13.8 volts, it waits 10 seconds, then activates the failsafe. This is why you didn’t see it activate immediate. You can adjust this with parameter BATT_LOW_TIMER (wiki)

  • If your battery went from 13.8 volts to 12.8 volts in 12 seconds, the battery is definitely shot. Assuming you’re not running an underpowered system that over utilizes the battery.

I suggest taking a look at the ArduCopter wiki page on Battery Failsafes.

Ok, I will change my battery failsafe, thank for the links!

From the logs I’d agree with Matt, set:
And check your ESC settings and turn off any low voltage cutout, or at least make sure it’s a soft-cut or reduction in power.
If you’ve got a cheap battery charger, invest in a better one.

Also check that the voltage reading is accurate, mostly importantly at the lower end of the expected voltage scale like 12 or 14 volts. Dont calibrate it using a 16 volt source, use a known 12.0 or 14.0 source. This must be accurate.
In the logs the voltage does drop right away but it goes down at a fairly constant rate after that and only really drops off after 13.5 volts - this is why I’ve said to make sure the voltage measurement is accurate. I would have expected the voltage to drop a little faster if you’d exceeded the batteries capacity by so much.

Also set these:

MOT_BAT_VOLT_MIN should actually 3.2v x Cells. So for a 4S pack, that should really be 12.8 for proper scaling.

Good point, thanks Matt

Wiki say 3.5 * cell_count as a recommended value. So, is it 3.2 or is it 3.5?:

“Battery voltage compensation minimum voltage (voltage below this will have no additional scaling effect on thrust). Recommend 3.5 * cell count, 0 = Disabled”

So we have a documentation conflict to fix. The tuning process wiki page says 3.3 volts & cell count, which is what i was thinking of when I said 3.2. Where did you see 3.5?

@Leonardthall, what say you on the MOT_BAT_VOLT_MIN parameter? 3.3v or 3.5v times the cell count?


If MOT_BAT_VOLT_MIN is a little lower than what the battery voltage can realistically be expected to drop down to (and the craft still flies) then it may not matter - I havent checked through the code though.

There are two separate issues here battery failsafe parameters and MOT parameters.

Failsafe settings:
FS_BATT_VOLTAGE 3.5 or 3.6 * cell count
(sag compensated with later versions too)

Thrust compensation settings:
MOT_BAT_VOLT_MAX 4.2 * max cell count
MOT_BAT_VOLT_MIN 3.3 or 3.5 * min cell count

I personally use 3.5 volts per cell and sag compensation enabled.

Thank Leonard, I will config those values. I’m reading your tunning guide right now, what a wanderful info!!!

May I ask what autotune algorithm do you implement in AP?

I don’t know if it has a name or if something similar has been done before. I designed it from scratch.