A Voltwatch is NOT a loaded tester. It only tells you the surface voltage of the pack. It won't tell you anything about the health of a battery pack under load. You need a loaded tester and a battery cycler to do that.

However, a Voltwatch WILL tell you very well, that the ignition is powered on, which is a very, very good thing. And if you started the flight with everything in the green, and you land and everything is in the red, then you know there's a problem! Another option is to use the MPI charge/switch that has the built-in LED's for your ignition. Clip the wire and set it to 6V operation if you are running lithiums.