First let me say that many people think they are worthless and depending on how you use them, I may agree.

I use them on both my plane and my ignition.
I check the reading when I land as I wiggle the sticks.
The ignition voltage is the loaded voltage when the engine is running. I feel safe with that.
The RX voltage to me is safe so long as only the green lights are lit after landing.
I never use them with NiMH because the discharge curve for NiMH is too flat in my OPINION>