I have two B&C sealed 12 AH batteries in my 320. They are made by Hawker
Energy and are terrific batteries.  They are both mounted on the firewall,
one on the engine side and one on the cockpit side. Neither is in a box.
The batteries are normally connected in parallel by a DC bus tie relay. Both
start the engine and both are charged by the alternator. In the event of an
alternator failure, I can open the bus tie. With the bus tie open, the B
battery powers only the essential bus (GPS/COM,  fuel transfer pumps, and
cockpit flood lights). This keeps the engine running, allows me to navigate,
communicate and see the panel. It is sufficient to get me on the ground in
night IFR. With only those few items on the essential bus, the B battery
will outlast the fuel.

All other electrical stuff is on the normal bus and would be powered by the
A battery in the event of an alternator failure. The two 12 AH batteries
weigh only very slightly more than a "normal" 25 AH battery and give you
considerably more versatility and safety. By the way,  either little 12 AH
battery can easily start the engine by itself.

This system has worked out well and if I were building again (perish the
thought) I'd build it the same.  It was certainly a system that we were glad
to have on our recent trans Atlantic flight.


Bill
harrelson@erols.com
N5ZQ  LNC2  O-320  VA42  Fredericksburg, VA