Battery required?

[Stoneage]

Great thread for thought.
I am gathering we are discussing the electrical system for an ALTERNATOR equipped C-172 rather than a generator system. All of the Lycoming powered C-172's use alternators and most light planes built in the last 40 years have similar systems. Some aircraft (notably military fighters) have no battery at all but their electrical systems are wildly different and not relevant to the question at hand.

We should define battery failure.
If a battery is "dead" that doesnt really mean it failed in the sense that it is still forming a circuit but lacks adequate voltage ( a 12V lead acid battery should should produce 13 to 14 volts across the poles unloaded, this will vary with battery condition and to a lesser degree temperature. If said battery is only producing 8 volts it is definitely "dead" but still forms a curcuit) A battery with an internal failure, such as plate separation would cause an "open" and thus the circut would not be complete. These are two completely different situations.

Now for the part I am not sure about: So if you had a dead battery and started the plane with external power that should provide power to the alternator field and it would produce power as long as it was not interrupted and the engine was turning (even after removal of external power). If you had a failed battery and thus an open circuit instead of a dead battery would the field continue to be energized, I think it would. But, in either case, if power to the field were interrupted you could not re-engergize it in flight.

[flyguy]

Originally Posted by Torqued

Usually if the battery is completely dead then it does not have the power to close the battery master relay and will not come on line. It is usually wired so only battery voltage will close the relay so even if another power source is on line, the battery will not be.
I've only seen self exciting generators (3 phase AC generators) on larger aircraft. Then there's DC generators, a different animal.

The BE76 has self-exciting AC alternators.

[Torqued]

Now for the part I am not sure about: So if you had a dead battery and started the plane with external power that should provide power to the alternator field and it would produce power as long as it was not interrupted and the engine was turning (even after removal of external power). If you had a failed battery and thus an open circuit instead of a dead battery would the field continue to be energized, I think it would. But, in either case, if power to the field were interrupted you could not re-engergize it in flight.

In the above situation the alternator would produce power but without the battery in the circuit to help stabilize it there could be some wide voltage swings when different circuits are turned on or off. The voltage regulator would attempt to control it but it is designed to have a battery in the circuit.
I have seen a modified automotive alternator circuit work this way but the application of a big electrical load (landing lights, gear retraction, etc.) can cause a large enough drop in system voltage to lose alternator excitation. If you're lucky enough to have the self exciting alternator that may not be a problem but the wide system voltage swings could do ugly things to avionics. Especially the spikes in voltage after turning off a large electrical load.
So you could get the alternator on line at startup with external power but it would be tricky keeping it on line with electrical load changes. Then even if it stayed on line the wide voltage variations could make it a very expensive flight after fixing the electronics that are fried.

[Torqued]

Forgot to mention, in the case of a "dead" battery (< 8 volts), yes it would absorb the spikes and the system would probably act almost normal IF you could get the battery on line. The alternator would charge it at a rate that would probably bring its voltage up to near normal. Of course a failing battery brings with it another list of potentially ugly problems.
A battery with an open circuit is just acid filled ballast.