Sorry, yes…apart from the 3.8v/cell = 50%, the numbers are under load.
Huh, so there is
Sory, http://ardupilot.org/copter/docs/parameters.html#batt-fs-voltsrc-failsafe-voltage-source
I cannot find how it calculates the sag?.
Sag will vary greatly depending on battery type and load.
I set up my failsafes based on under load values. If you go below 3.3v/cell, under load, you will wear down the cell quickly, below 3v/cell, you can kill it.
First few flights, I rarely use more than 50% of estimated usuable capacity. I then go home, look at the discharge curve, vs expected and then go 75% next flight.
As a general rule of thumb, I expect usable capacity to be ~80% of rated capacity. Some batteries, like the Multistars, you were lucky to get 50%, even if you stayed well under the batteries claimed C-rating. (The “cheaper” tattus weren’t much better).
You also have to anticipate that as voltage goes down, current goes up and the battery will discharge faster.
I now run li-ion, so my values are quite different, but as a starting rule for lipo, I would have 3.7v/cell as my first failsafe (RTL if you don’t trust the telemetry!). and 3.4v as Land.
Then adjust as needed.