P3S lucky to be back home w/critical low power init @ indicated 56% battery capacity level

Lanse Adams · Apr 10, 2019

Questions: Why did battery voltage drop from 15.109 to 13.59 at 5.4sec into flight, with capacity indication still 61%? Why was critical low power/auto landing initiated at 1min 36.9 sec. with no change in battery voltage/capacity parameters? I was able to pilot bird home using full vertical joystick, narrowly missing tops of 125 ft. tall trees. How is battery capacity determined, is it separate from actual voltage readings? Could battery connections have been faulty? Battery was properly seated. Subsequent flight with fully charged battery was uneventful, but I did not fly long enough to drop battery cap. below 74%. See flight log DJI Flight Log Viewer - PhantomHelp.com.

LarBear360 · Apr 10, 2019

Lanse Adams said:
Questions: Why did battery voltage drop from 15.109 to 13.59 at 5.4sec into flight, with capacity indication still 61%? Why was critical low power/auto landing initiated at 1min 36.9 sec. with no change in battery voltage/capacity parameters? I was able to pilot bird home using full vertical joystick, narrowly missing tops of 125 ft. tall trees. How is battery capacity determined, is it separate from actual voltage readings? Could battery connections have been faulty? Battery was properly seated. Subsequent flight with fully charged battery was uneventful, but I did not fly long enough to drop battery cap. below 74%. See flight log DJI Flight Log Viewer - PhantomHelp.com.

I'm no expert and this has never happened with me, however, I have read that taking off without a fully charged battery can lead to inaccurate readings. Was that your first flight of the day? It's possible you took off with less power than indicated.

sar104 · Apr 10, 2019

Lanse Adams said:
Questions: Why did battery voltage drop from 15.109 to 13.59 at 5.4sec into flight, with capacity indication still 61%? Why was critical low power/auto landing initiated at 1min 36.9 sec. with no change in battery voltage/capacity parameters? I was able to pilot bird home using full vertical joystick, narrowly missing tops of 125 ft. tall trees. How is battery capacity determined, is it separate from actual voltage readings? Could battery connections have been faulty? Battery was properly seated. Subsequent flight with fully charged battery was uneventful, but I did not fly long enough to drop battery cap. below 74%. See flight log DJI Flight Log Viewer - PhantomHelp.com.

Almost certainly because you launched with a partially charged battery. The battery % is not reliable at all because the voltage behavior under load is different. So if you look at the usual smart battery data it looks like it should be fine:

But if you look at the actual cell and battery voltages you can see that as soon as there is significant current demand the voltage collapses. Anything less than 3.3 V is critically low.

Don't do that.

Meta4 · Apr 11, 2019

Lanse Adams said:
Questions: Why did battery voltage drop from 15.109 to 13.59 at 5.4sec into flight, with capacity indication still 61%?

To add to what's been said already .. you need to look at the battery voltage rather than the % indication which will be false when the battery is not fully charged.
Your voltage was quite low at the start (as shown in the Phantomhelp report).
But within 6 seconds it had fallen below 3.3V per cell (critical low voltage).
Once that's happened, it all downhill from there.

Lanse Adams · Apr 11, 2019

Thanks for all the replies. I agree that taking off with a partial battery charge is ill advised. I was not aware that battery capacity readings may not be accurate for a partially discharged battery. I believe there is an setting in DJI GO that will display votages on the screen during flight.