The most likely cause was a bad Voltage Regulator or a dirty connection. You should see almost no load when the engine is running you will see the swing as the beacon goes on and off. Pull the spade connectors on and off the master switch to clean them and have your mechanic check the VR.
Put a good charge on the battery and with a new VR, that should fix the problem. You can give Ken (AirModsNW) a call or me.
Cheers
Roscoe