The thought that backup electrical power is overly complicated or
unreliable probably scares some builders away from a modern all-electric
plane. The backup electrical system on my Lancair is uncomplicated and
reliable. It can be fully tested on the ground at any time by simply
switching it on and letting it run the connected loads for a few hours.
Whether or not you like my overall system, you may want to adopt some
features, such as the battery's location and installation method. The entire system consists of only three components:
1. A backup battery installed on the right side of the nose gear tunnel,
forward of the rudder pedals (check to see how much room is available in
this area on your aircraft, there's enough to accomodate at least a
medium-size battery).
The battery need not be physically large. Should backup power ever be
necessary, it will be required to operate critical loads totaling only
5-10 amps for 2 to 3 hours at most. Even if you had enough gas onboard to
continue flying for a longer time, it would be foolish to press your
limits. By definition the A-H (amper-hour) capacity of a battery is it's ten hour
capability at rated amperes. A 12 A-H battery, for example, will deliver
12 amperes continuously for at least ten hours, if it's fully charged. I
installed a Concorde RG-12 as a backup four years ago, but it's no longer
manufactured (A Lifeline GPL-1300 (marine grade) is their current
equivalent). Now, I'm using a Sears Die-Hard. There are many batteries you
could choose, but I would go with a modern RG (recombident gas) sealed
lead-acid type
To install it, I first fastened a 6" length of 2" x 2" aluminum angle
(purchased at a local hardware) to the side of the nose gear tunnel
(approximately 4" above the subfloor) using 3/16 flat head aluminum pop
rivets. In order to successfully "pop" the rivets and for load spreading,
use 3/16" area washers (AN370-16) on the inside of the nose gear tunnel.
The battery then rests on the aluminum angle and is secured against the
tunnel by means of a 1" wide nylon strap (purchased at a local marine
supply and made for the purpose). The strap is supplied with two small
stainless steel brackets to allow it to wrap around the battery. Attach
the brackets to the nose gear tunnel forward and aft of the battery, again
using pop rivets and area washers.
 
2. A power diode connecting the backup battery to the main (charging) bus.
It has two functions:
    a.) Provides a path for charging current from the main bus.     b.) Provides isolation for the backup battery (preventing backup
current from flowing back to the main bus).
I used a conventional silicon power diode, which has an inherent voltage
drop of approximately 0.7 V DC in the forward direction. This lowers
charging voltage to the backup battery and if your main bus voltage is
less than 14.0 V DC, the result would be less than full charge on the
backup battery (Concorde states that their testing shows that as little as
13.5 V DC will maintain full charge on a lead-acid battery). However, as
Ed Armstrong just posted, a Schottky diode, which has a voltage drop
ranging from 0.2 V to 0.3 V (at the charging currents we would experience)
can be used, thus eliminating the possibility of undercharging the backup
battery due to low voltage. By the way, I contacted VMS and was informed
that the VMS 1000 accuracy is +/- 1% for all instruments, which equates to
0.14 V at 14.0 V DC for the voltage instrument.
As pointed out by Jim Frantz, another possible fault would be shorting of
the diode, thus connecting the backup directly to the main bus, which
would drain it in a relatively short time. Although such an occurance was
reported by George Stevenson in the LNN several years ago, I believe this
to be a rare event provided that the diode is suitably rated. 3. A DPDT (double pole-double throw) switch rated at least 10A. This is
about as simple and reliable an electrical device as exists. It has three sets of terminals.    a) The center terminal is wired to the device(s) requiring backup
power.    b) Power from the main bus is connected to one outer terminal.
   c) Power from the backup battery is connected to the other outer
terminal.
Whichever position the switch is in determines whether power is fed to the
device (ignition, gyros, etc.) from the main bus or the backup battery.
Admittedly, it would be impractical to connect several devices (wires) to
a terminal. I connected directly to the switch, but you could make an
"essential" bus for connection of several devices.

Bill Rumburg
N403WR  (Sonic bOOm)