This is a feature request:
On the A2 flight controller, there is a two level battery voltage warning. At the first threshold, the status LED turns yellow simply to indicate to the pilot that the voltage is low. At a second threshold, the status LED turns red and a failsafe (either RTL or Land) is engaged.
This is super useful when you are flying without telemetry, but you still want to keep track of your batteries.
I would love to see a similar feature for APM! Especially because I'm guessing it would be relatively simple to implement. If i had a few more hours in the day, I would learn enough to code it myself, but you know how these things go... :/