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Old 04-17-2018, 08:32 AM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,440

Originally Posted by Nihon_Ni View Post

I was taught years ago that Cessna used the Bendix switch for the master, so you cannot have the alternator on without the battery on first, as a safety measure.
Cessna has used a split master switch since at least the 70's, that mechanically prevents disconnecting the battery while leaving the master still on.

I am aware of over voltage events that have occurred and caused failure of a lot of equipment mounted in the panel, while experimenting with disconnecting the battery with the engine running and the alternator on line.

This was with older vintage externally regulated alternators. Point is, that depending on what your airplane has for an alternator, regulator and over voltage protection, disconnecting the battery on an operating system could be very expensive.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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Old 04-19-2018, 06:13 AM
togaflyer togaflyer is offline
Join Date: Apr 2013
Location: Cleveland Ga
Posts: 435

The alternator will sustain the power needed. When you start, the alternator output will have higher amps until the battery has been replenished, then you will see the amps drop down to meet your systems demand. The battery is mostly for starting and a power reserve. Power from the battery is also needed, on start up, to energize the alternator field. After that the battery has done its work. I would not disconnect the battery with a running engine because possibly, it could damage the alternator. The current produced needs to go somewhere, even if the battery is dead it still absorbs the current of of the alternator.
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Old 04-19-2018, 08:08 AM
rv7charlie rv7charlie is online now
Join Date: May 2006
Location: Pocahontas MS
Posts: 2,698

Y'all (Southern thing) need to look at how *your* plane will handle the various conditions you're discussing. A battery failing open is pretty much a non-event in most planes. There just isn't that much 'ripple' in the alternator's output, especially at low to medium current demand. Worst thing you (might) get is a little extra 'alternator whine' in your audio system.

Losing a master solenoid could be very different. How is your plane wired? Assuming it's wired like the Aeroelectric connection diagrams, if the alternator B lead is on the load side of the master solenoid (and you don't cycle the master switch), then the bus stays alive and the alternator stays alive. On the other hand, if you cycle the master switch, you'll remove power from the regulator, and the alternator will shut down and the bus goes dark.

If you're using an internally regulated automotive alternator, you need to follow the little lines in your wiring diagram and ask what happens in each failure mode. (Battery failing open is a non-issue here.)
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