II have a PC680 main battery, and a 7.5AH aux battery for powering ALL the avionics during engine start. The two batteries (both lead acid) are tied together during normal operations So both are equally charged - no diode drops), but automatically separate (via an under-voltage sense circuit driving a contactor) if the main bus voltage goes below 12 VDC, and during the time the engine starter is being engaged. Both batteries are used separately to power the essential avionics. I ALWAYS turn ALL avionics on prior to engine start, and none reset during engine start. I have manual switch operation of both battery contactors as well (OFF,AUTO, ON).. Both batteries are above the rudder pedals inside the cockpit, accessible via inspection covers on the front deck. If anyone is interested in the circuit design, I can post a PDF file of the schematic....