Rainier,
Looks like you could sell three of these things today if you can give us a good solution for backup power????
OK, that's an easy one...
Well, many of these units are used for backup purposes on larger aircraft or as primaries on smaller types.
One big factor is the really low power consumption of these things and low voltage requirements.
This means, in a nutshell, you could use a small 9V transistor radio battery to power these things quite happily for perhaps close to 10 hours or so (depending on the battery chemistry kind).
So, one way (there would be others of course) would be to stick a 9V battery in a holder from Radio shack (perhaps behind the panel where you can get to it), then use two diodes and a small switch. One diode connectes the power to the main bus, the other connects via the small switch to the 9V battery.
In practise, you would do your preflight like such: Main bus off, backup switch on - check to see if instrument works OK, then switch on main bus.
The higher voltage from the main bus would then stop any current flowing from the 9V battery - but if the bus goes down, the ALT-1 would continue to function as if nothing happened and will be visible even at night due to the backlight.
The switch prevents the battery from running down when you're done flying. If you have a main power switch with a spare set of contacts you could use those - but then you would need some other means to check if the backup is working when main power is down.
Another option would be to use a rechargeable 9V battery (such as a NiMh) with another diode from the main bus and a resistor in series to trickle charge the battery when the main bus has power.
For a more comprehensive backup instrument our Flight-II has become quite popular - but it needs a 3.1/8" hole. It's almost a small EFIS for next to nothing and even can give you engine RPM and fuel related stuff like flow and level apart from all primary flight stuff.
The Flight-II BTW is soon going to be available in a new, round housing as well as are all other maxi-singles. The new range will be known as "Velocity".
Rainier
CEO MGL Avionics