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Backup Battery Switch

iamtheari

Well Known Member
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.
 
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.
 
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)

Mark
 
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?

Charlie
 
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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.
 
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.
 
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.

FWIW,

Charlie
 
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.
 
Ari,

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. I?m 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.

Carl
 
Ari,

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. I?m 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.

Carl

The battery switch as set out in my OP above does not violate the objective. The backup battery is on anytime the main is on, and connected to the dedicated backup power pins on the Garmin equipment it powers. If the main bus dies, that equipment should automatically run from the backup battery with no pilot intervention. Possibly I was not clear on the operation. The switch positions would be OFF (both batteries disconnected), AUX (backup battery activated but main contactor open), and MAIN (backup battery activated AND main contactor closed).
 
I have a similar setup. I think I understand your last post but a drawing would be better to understand what you are trying to do. On my system, I have a traditional dual Cessna switch that control 2 separate batteries, a separate switch that turns on and off my back up battery for the G3X system and an alternator switch. The backup battery switch is the first thing I turn on and the last thing that I turn off. It needs to be On to re-charge the backup battery (at least in my set up it does). The only thing I will emphasize is this, if I'm understanding you correctly you want the switch to be OFF and for it to still work as outlined above... It needs to be On for it work this way and also to charge the backup battery... otherwise, why would you even have a switch to the backup battery?
 
What Charlie and Walt said. Two switches, physically separated or different. You want to be able to kill the master while bouncing around inside a cloud at night.

And label them "Master" and "Backup Battery" rather than the generic "Aux". You built it, so you don't need any labels at all. The whole point of labeling is to make it obvious to the next guy.
 
I have a similar setup. I think I understand your last post but a drawing would be better to understand what you are trying to do. On my system, I have a traditional dual Cessna switch that control 2 separate batteries, a separate switch that turns on and off my back up battery for the G3X system and an alternator switch. The backup battery switch is the first thing I turn on and the last thing that I turn off. It needs to be On to re-charge the backup battery (at least in my set up it does). The only thing I will emphasize is this, if I'm understanding you correctly you want the switch to be OFF and for it to still work as outlined above... It needs to be On for it work this way and also to charge the backup battery... otherwise, why would you even have a switch to the backup battery?

I stink at drawings. Maybe I stink at descriptions, too. The switch would have three positions. From bottom to top, they would be: OFF, which disconnects both the master and the backup battery. BACKUP, which connects only the backup battery. And MAIN, which connects both the backup battery and the main battery. It would not be possible to turn the backup battery off while the main battery is on. It would be possible to turn the main battery off while the backup battery is on.

But the point immediately below yours is quite valid: In turbulence, it would be quite easy to accidentally push the switch full down and shut off the entire airplane while trying to switch to backup power in the event of a main power bus failure. Separate switches are probably safer for that reason. In my plane, when all else is equal, safety makes the decision.
 
Like I'd mentioned in my previous post, check out the Honeywell 2TL1-31 switch. It's a very heavy duty double pole/three position switch with a lockout preventing you from inadvertently moving the switch into the off position, but it can easily be toggled between positions two and three. I'll defer to the rest of the experts with regards to your overall architecture of your switchology the do's and don'ts, but this Honeywell switch with the lockout might just do the trick and accomplish what you're trying to do.

Mark
 
Like I'd mentioned in my previous post, check out the Honeywell 2TL1-31 switch. It's a very heavy duty double pole/three position switch with a lockout preventing you from inadvertently moving the switch into the off position, but it can easily be toggled between positions two and three. I'll defer to the rest of the experts with regards to your overall architecture of your switchology the do's and don'ts, but this Honeywell switch with the lockout might just do the trick and accomplish what you're trying to do.

Mark
Thanks for the part number suggestion. Switch part numbers are always fun to look through because there are a million different configurations. Honeywell's document about the TL series is no different.

Most likely I will simplify this to separate switches, but anywhere that I do use a 3-position switch, I now know that there are lockout switches available to use.
 
Thanks for the part number suggestion. Switch part numbers are always fun to look through because there are a million different configurations. Honeywell's document about the TL series is no different.

Most likely I will simplify this to separate switches, but anywhere that I do use a 3-position switch, I now know that there are lockout switches available to use.

I think this is the switch Mark was referring to:

https://www.steinair.com/product/locking-toggle-switch-dpdt-ononon/
 
Honeywell toggle switches

Yeah, that same Honeywell 2TL1- switch comes in about ten different configurations based on lockout position or no lockout at all, momentary or not, all three positions on, one position off, etc. etc. The last number that you put in after dash (2TL1-?) determines the configuration. Here's their pdf brochure of all their toggle switches and all of the different configurations that you can get them in. https://sensing.honeywell.com/honeywell-sensing-micro-switch-tl-toggle-product-sheet-005430-3-en.pdf You just need study them a little bit to make sure you're getting the configuration that you want. There?s quite a few places online that you can order them from.

Mark
 
Yeah, that same Honeywell 2TL1- switch comes in about ten different configurations based on lockout position or no lockout at all, momentary or not, all three positions on, one position off, etc. etc. The last number that you put in after dash (2TL1-?) determines the configuration. Here's their pdf brochure of all their toggle switches and all of the different configurations that you can get them in. https://sensing.honeywell.com/honeywell-sensing-micro-switch-tl-toggle-product-sheet-005430-3-en.pdf You just need study them a little bit to make sure you're getting the configuration that you want. There?s quite a few places online that you can order them from.

Mark

Thank you for sharing that catalog link.

Charlie
 
Here at SteinAir, the combination we see most often is a Master switch for the primary battery and primary alternator, usually the aforementioned locking toggle, which we stock at SteinAir. If needed, we add a second switch for a second alternator field and if there are two batteries and they are not in parallel, another battery switch and a second contactor. With two separate batteries things get complicated so I won't go into that here. If you have two alternators we advocate running them both at the same time, with the backup alternator regulated .5 volt or so lower than the primary. The backup will then "idle" with little or no excitation until the bus voltage drops below it's regulated value. It will then pick up the load, and if you set an alert between the primary and backup voltage your EFIS can show "Primary Alt Fail" or something similar. There is absolutely no point in leaving a second alternator turned off in flight.
If you have an avionics backup battery such as TCW's IBBS, the drill is usually to turn it to the On or Armed position first, make sure it powers the connected items. Then you proceed with the normal power-up and start procedure. The IBBS will also prevent an EFIS reboot ("brownout") during engine cranking. The IBBS switch stays in ON or Arm for the whole flight. There is also no point in trying to run the IBBS at the same time the main battery/alternator is powering the bus, as it only comes on when the bus voltage is low. The idea is to use the main battery first, then the IBBS. Our EFIS systems only draw about 7-9 amps when fully powered so between the main battery and the IBBS you can fly a long time and still have your EFIS and EMS.
 
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Here at SteinAir, the combination we see most often is a Master switch for the primary battery and primary alternator, usually the aforementioned locking toggle, which we stock at SteinAir. If needed, we add a second switch for a second alternator field and if there are two batteries and they are not in parallel, another battery switch and a second contactor. With two separate batteries things get complicated so I won't go into that here. If you have two alternators we advocate running them both at the same time, with the backup alternator regulated .5 volt or so lower than the primary. The backup will then "idle" with little or no excitation until the bus voltage drops below it's regulated value. It will then pick up the load, and if you set an alert between the primary and backup voltage your EFIS can show "Primary Alt Fail" or something similar. There is absolutely no point in leaving a second alternator turned off in flight.
If you have an avionics backup battery such as TCW's IBBS, the drill is usually to turn it to the On or Armed position first, make sure it powers the connected items. Then you proceed with the normal power-up and start procedure. The IBBS will also prevent an EFIS reboot ("brownout") during engine cranking. The IBBS switch stays in ON or Arm for the whole flight. There is also no point in trying to run the IBBS at the same time the main battery/alternator is powering the bus, as it only comes on when the bus voltage is low. The idea is to use the main battery first, then the IBBS. Our EFIS systems only draw about 7-9 amps when fully powered so between the main battery and the IBBS you can fly a long time and still have your EFIS and EMS.
Just saw your post today. Thanks for posting it. What is odd to me is to see planes with the IBBS switch located in such a place on the panel that it is not a smooth flow to turn that on as step 1 in the before engine start checklist, as it seems it ought to be.
 
Just saw your post today. Thanks for posting it. What is odd to me is to see planes with the IBBS switch located in such a place on the panel that it is not a smooth flow to turn that on as step 1 in the before engine start checklist, as it seems it ought to be.

See post #5...
 
I have occasionally wondered if there is any point in using the IBBS to also power the Standby Alternator reg and field (in parallel, or maybe by some diode or bootstrap switch function).

Probably overthinking options (it?s 115f in Phoenix so I left the hangar to muse wiring).

Could there ever be a case where some independent means of kicking on an alternator might be useful? Sorta rare I suppose
 
Rare event

I have occasionally wondered if there is any point in using the IBBS to also power the Standby Alternator reg and field (in parallel, or maybe by some diode or bootstrap switch function).
...

Could there ever be a case where some independent means of kicking on an alternator might be useful? Sorta rare I suppose
Yes, this might be helpful under some very rare circumstances, like main battery failure (wiring problem?) and then during troubleshooting you somehow stop the alternator completely and it needs a bit of juice to get started again. IFR guys might want to consider this scenario during their wiring design phase since a black cockpit on a dark and stormy night is not a fun place to be.
 
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