Bat.volt, bat.voltr, Fvas -- different voltage values?

While checking a recent log using the excellent plot.dron.ee site, for a flight when I had a battery failsafe condition, I noticed there are 2 named values:
bat.volt & bat.voltr which are approx. 1 volt different.

I can see the FasV voltage on my radio which is provided via Frsky passthrough telemetry.

I am running 3.6.11 on Pixhawk/Chibios with a Mauch PM.

Can someone pls explain the origin/meaning of these 3 voltage values and why they would not all be the same?

Battery voltr is the estimated resting voltage. This is calculated from the estimated battery resistance. The resistance is calculated by seeing how the voltage drops with current draw.

The voltage you see on your pass through telm is probably the raw voltage.

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So the voltr value is not much use when flying :slight_smile:

Which voltage value is used by the flight controller when comparing with the battery failsafe minimum and does it need to below that minimum for a particular length of time before it triggers the action e.g. RTL?

I think some people of are of the view that it is more useful because it reduces the voltage sag that would happen due to a sudden throttle increase for example.

There are parameters for that, BATT_FS_VOLTSRC and BATT_LOW_TIMER

http://ardupilot.org/copter/docs/failsafe-battery.html#advanced-settings

Tx for the link to advanced settings. I did not know about those extra params introduced in 3.6.