I should have added that my desire is to convert from 6 to 12 volts in order to use a newer accessory in a vintage car that runs a 6 volt battery.  Input voltage would be somewhat over 6 volts depending on the level of charge in the battery and the output of the alternator.  Likely in the 6.3 to 6.8 range.  Output should be right about 12.3 – 12.6.

Source power will be on when the car ignition is on but the load for this unit will only be present when the accessory (a small fan) is needed.