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Old 01-11-2019, 10:27 AM
iamtheari iamtheari is offline
Join Date: Nov 2017
Location: ND
Posts: 344
Default Backup Battery Switch

I started planning my RV-14 panel starting about a month before I ordered the empennage kit. It's an ongoing obsession to perfect the panel, especially the switches, to make every possible situation a left-to-right flow that is not error-prone. That leads me to ponder the options for the switch to control the backup battery (I plan to have a single iBBS that will feed the essential parts of the G3X Touch-based panel along with alternate trim and alternate flaps switches, which allow you to bypass all electronic control of the pitch trim and flap motor and connect them through panel-mounted momentary switches fed directly from the backup battery).

It seems that most people have a main battery switch and a separate backup battery switch. My thought is to use a single switch with 3 positions: OFF, AUX, MAIN. When in the OFF position, both the main and backup batteries will be disconnected. In the middle AUX position, the backup battery will be activated. In the MAIN position, both the backup battery and the main battery contactor will be activated. This way saves a switch and ensures that you can't turn on the main power of the airplane without having the backup battery on and ready. You also are less likely to forget the backup power on when you turn off the plane. The arguable disadvantage is that you have to touch the same switch twice during the before-engine-start check list: Once to turn on backup power and observe the EFIS booting and a second time to turn on main power.

Please poke holes in my idea before I spend the $43 on a SP3T MS25201-4 switch instead of $24 to get two SPST MS35058-22 switches.
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Old 01-11-2019, 11:03 AM
rv8ch's Avatar
rv8ch rv8ch is offline
Join Date: Feb 2005
Location: LSGG
Posts: 2,573
Default Alternator switch

What's your plan for the alternator switch? Normally I've seen off/bat/bat+alt

What happens when you want to run both the ibbs and the normal battery?

If you turn off your battery while the alternator is running, that can be bad, it seems.

I have a separate switch for the ibbs.
Mickey Coggins
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Old 01-11-2019, 02:48 PM
iamtheari iamtheari is offline
Join Date: Nov 2017
Location: ND
Posts: 344

Originally Posted by rv8ch View Post
What's your plan for the alternator switch? Normally I've seen off/bat/bat+alt

What happens when you want to run both the ibbs and the normal battery?

If you turn off your battery while the alternator is running, that can be bad, it seems.

I have a separate switch for the ibbs.
I'm torn on single or dual alternators. If I go dual, then I plan to have a 3-position switch: ALT1 / OFF / ALT2. This would isolate the two alternator positions and prevent them from both being energized at the same time. The alternative is a main off / battery / alternator switch like you and many others have and then a second alt1 / alt2 switch among my "abnormal operations" switches. If I have a single alternator, then I can combine the battery and alternator into a single switch assuming I move the backup battery to its own switch.

The 3-position switch idea I mentioned would keep the backup on in all positions other than OFF, so if the main is on then the backup is on. The only position not possible would be main on and backup off.

The main off, battery, alternator switch is a common solution, maybe for the reason you mentioned. I'm not sure if it's actually bad to have the alternator switch on with the battery master off (because the alternator field would not be energized and, even if it were, the alternator B+ output is tied to the battery and not to the rest of the electrical system). I guess it makes more sense than the combined battery and separate alternator switch idea I started the discussion with, because it makes sense to have the main on and the backup battery off if the backup battery is causing problems, but it never makes sense to have the alternator switched on (but doing nothing) and the battery master off.
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Old 01-11-2019, 03:20 PM
rv7charlie rv7charlie is offline
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,871

Combined ALT1 / OFF / ALT2: See earlier posts about single point of failure taking down everything.

Empirical evidence has shown that having two alternators online at the same time has little if any detrimental effect. Traditional prohibitions/elaborate cross connected regulators/etc were driven by the nature of *generators*.

Similar situation for no-battery operation. As long as the regulator has a voltage to reference (the bus), modern alternators are fairly stable even without a battery. Empirical evidence suggests no more than a volt or two of variation; less than you get between alternator on and off (failed) in a regular system.


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Old 01-11-2019, 03:34 PM
BMC_Dave BMC_Dave is offline
Join Date: Mar 2017
Location: Seattle, WA
Posts: 288

B&C make a standby alternator voltage regulator that monitors bus voltage, if your main ALT goes down then it automagically brings up the standby alternator so you don't need to worry about switching (you leave both ON for normal ops). This is my plan.
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Old 01-11-2019, 03:56 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
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Almost impossible to respond to your question without a basic power distribution drawing.

One comment - recommend a design objective that, with no pilot action following any practical single point failure, you do not loose all the panel. Im thinking pilot action to switch in a backup battery violates this objective.

On the standby alternator issue, as pointed out the standby alternator is alway on. The standy alternaor picks up load as buss voltage drops after the loss of the primary alternator (assuming it was properly configured). There is no need to have a switch for either alternator other than a breaker you can open from time to time to verify the standby alternator functions properly.

Side note - I would recommend your design always have at least one battery on either alternator output.

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Old 01-11-2019, 11:22 AM
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Mark33 Mark33 is offline
Join Date: May 2010
Location: Baton Rouge, La.
Posts: 705

I like the idea of having switches do double duty. Although I don't have much feedback to give you on your particular architecture, I would encourage to look at the Honeywell TL switches. The two pole/three position 2TL1-10 is an outstanding switch. You can also have it configured with a lockout to prevent you from inadvertently turning it off if you'd like. (2TL1-31)

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Old 01-11-2019, 01:14 PM
rv7charlie rv7charlie is offline
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,871

For a general 'philosophy' sort of view, a couple of basic ideas are to always have a plan B, and try not to have a single point of failure that can take everything down. The 2nd point obviously requires some compromise (one set of wings, and for most of us, one engine). But for things over which we have some control, we design *not* for what's likely (or unlikely) to happen, but for what happens *if it does*.

So, what happens if that $40 switch *does* fail? If there's a plan B, no problem. If there isn't a plan B, perhaps further planning and consideration is needed. If two $10 switches would enable plan B, I'd find that hard to resist.

edit: Another data point: with a 3 position switch, I'd be thinking about what I'd do in the 'heat of battle'. If the master needs to go off in a high stress situation, how likely would I be to slap the toggle to full-off, instead of stopping at the middle position?


Last edited by rv7charlie : 01-11-2019 at 01:18 PM.
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Old 01-11-2019, 02:00 PM
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Walt Walt is offline
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,305

I ALWAYS do a seperate backup battery switch and prefer to locate it somewhere other than next to the master or other switches. What Charlie said, if there is smoke in the cockpit, master off is usually the first step, don't want to inadvertantly kill my flight instruments and make things worse.
Walt Aronow, DFW, TX (52F)

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