I view a second battery as a way to maintain radio and transponder contact with ATC for as long as possible.

 I start up using both batteries and when the charge rate reaches a steady minimum I take one off line. I used MOV's on the contactors to absorb spikes as they're switched off since the avionics are kept on throughout.

 If anything happens to the alternator or the other battery I know I have a conservative (and easy to figure) 30 Amp-hours before voltage drops to 10V. The volt/amp shunt is wired to show battery charge/discharge rather than alternator load. If the alternator dies it then shows battery drain, making load shedding and time remaining calculations easier.

(This is a 14V system using RG-35AXC Concordes.) At 10V I would've lost autopilot, HSI, 530, audio panel, and Xponder. Until then I'd still have a GX-60 and an SL-30 for navigation and communication, turn coordinator, vaccuum A/I, altimeter and all engine gauges. I'd be able to use the compass rose on the 496 to substitute for a DG.

 There's no question a second battery is a heavier way to go. Part of the gain was offset by removing lead ballast. That wouldn't apply with a new airplane. For the rest, my wife says I need to lose weight....      -Bill Wade