Estimating flight time of an electric UAV

[Evan Schellenberg]

Article by Steve Loney, Lead Flight Tester at MicroPilot

This post is about flight time with lithium polymer battery technology and electric flight. In this example we will be trying to determine how long a UAV can fly for with a certain payload and takeoff weight.  To understand that we need to understand a little something about our batteries.

Some facts about lithium polymer (Lipo) batteries

• they slowly decrease in voltage output as they are depleted
• voltage will also “sag” when under heavy current loads(i.e. high throttle)  but bounce back when not under load
• only provide ~80% of their listed capacity before voltage drops too low to be useful (see picture at bottom

In our autopilot we can easily monitor voltage, however that isn’t the most accurate measure of how “dead” your battery is or how much longer you can fly.

For an accurate measure of flight time we will need to carefully measure how much current was used for any given amount of flying. With those numbers we can at least calculate flight time with theoretically perfect batteries:

So for one of our flights we used a total of 12.919 amps for 380 seconds of flight and our total battery listed battery capacity is 28 amps.

So, (380/12.919)X(28 x .8)

=29.414 (seconds per amp) x 22.4 amps

=658 seconds (10 min 58 seconds) total flight time before our battery is effectively depleted.

Again that is assuming perfect batteries and it still leaves us with the task of picking a warning voltage and deciding how much flight time we have before and after said voltage. The best way to pick that voltage and be reasonably sure of your flight times is rigorous real world testing of multiple charge and discharge cycles all with careful monitoring and recording of statistics.