gman,
mine did the same thing. If you will take the collar and battery case apart you will see two connections. Take the connections apart and leaqve them apart for 20 minutes. Put everything back and they will charge again. All you need is a small phillips screw driver. The 20 minutes resets the battery in some way. I've had to do this to two of my 30s. Now they work great.
Had the same issue with mine disconnected those hook ups as said above put them back together and they work fine