I took the trainer out Sunday for possibly the last session on skis since the snow is really melting fast. Had two great flights except for my usual prop-busting landings.....not real hard but hard enough to bump the front ski into the prop. I checked the receiver battery voltage ( a 1000 mah 4.8v ) and had 4.9v indicated with a load so I thought I was good to go. Fired up the plane did, my usual control check at full throttle, and was about to take off when I saw my horses run out into the pasture ( we had left the gate open ). So I shut down and gathered up the hay burners and got them back into the paddock. When I went back to start the plane for the last flight of the day I had no response at all on any of the servos. I checked the battery voltage and it was 0.79v. What would cause this sudden voltage drop? a short? I'm feeling mighty lucky that I didn't get airborne that last time but also a bit gun shy about batteries in general. Is there a way to get a back-up battery on board for this situation?

Thanks

Rufcut