I was shooting some approaches today in an almost brand new Cessna 172S at DTO when the volt light came on the annunciator panel. Two minutes later all of the avionics shut down, radios, GPS, transponder. Luckily it was severe clear out and my safety pilot had a portable transceiver with him, so we booked on back to the plane's home field which was about 5 minutes away. I'm not sure what the problem was. Everything looked fine at runup and there were no fuses that popped out. What's the deal?