Looking for a guesstimate of how long a B1B will run without an operating generator system. Would like to drive the truck in the upcoming 4th of July parade and not sure will have an operable genny system by then. Have been currently running the engine off a 12 volt battery, positive ground with a switched lead to the coil. Maximum run time has been 15 minutes or so to check operation of cooling system thermostat, etc. Anybody have a guess or experience with a failed generator system and still able to drive running off the battery only?