Something sometimes overlooked when designing EFI is what happens
when the system voltage drops off. With most car electrical systems the
voltage is regulated to 13.8 volts and the pulse width of
the fuel injection is based on that assumption. If the voltage 
drops off the injector does not open as fast. On my Ford system
the voltage is monitored by the computer and I assume the pulse
width is adjusted accordingly. Running on the back up battery
with no alternator the voltage will immediately be only about
11.8 volts and will drop off more as the battery is drained.

In a car closed loop system the computer gets feedback data from 
the O2 sensor and the system that can compensate somewhat for low
battery voltage. The Continental FADEC system sounds like it is
a closed loop system getting feedback from EGT/CHT.

In an open loop aircraft EFI system this is not the case. Fuel
is calculated from RPM, intake manifold pressure and intake air
temp.

Five volts is the common supply voltage that most digital circuits
run on. Most CPU's will stop working altogether as voltage regulators
in the computer stop regulating the CPU voltage to 5 volts.
This happens when the voltage supplied  to the CPU regulator 
from the back up battery drops below 8 volts.

At eight volts there are questions about the amount of fuel the
injectors will inject and the pressure from the fuel pump.

>From this it looks like a backup alternator is called for in
aircraft systems that are open loop.

Paul Lamar
 
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