I'm using TP 4s 3900 mah batteries in a 68 oz model and I'm trying out some props.

When I land, the Hyperion IOS Sentry shows that I have about 21% left in the battery.
However, when the battery cools down to room temperature it says I have 28% left.

Now I know it's not good to go below 20%, but is that measurement taken when the battery is warm straight after a flight or when the battery is cool?

Also, does the voltage variation from warm to cool mean that the batteries are getting tired?

Russell,
If you are using a castle brand speed controller, you should be setting the cutoff voltage to 3.O or even less and it should be set to a soft cutoff. This will prevent the motor from shutting down suddenly if the battery voltage goes too low for any reason at all. In CL a hard cutoff is our enemy. It WILL cutoff at the worst possible time in our event. I would rather destroy one battery by running it too low than lose a plane due to my motor shutting off. Replacing a battery is easier and cheaper than replacing your plane. Besides if the battery voltage is going too low it is already no good.