What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Backup battery with e-bus system

tcoverst

Active Member
Working on getting a schematic for wiring an e-bus with a backup battery system in my build. Does anyone have a schematic of an e-bus or some way of helping me think through so I can plan effectively for the switches and fuses I will need?
Tim
 
Get a copy of The Aeroelectric Connection.

It has a few different E-buss designs depending on how many alternators and batteries you are using.
 
Backup Power Example

My plan is to use a Seal Lead Acid (SLA) battery for backup power with a diode off main buss charging it. It is simple and inexpensive but the battery is heavy. The aux battery will power G3X, G5 and both electronic ignitions if the main battery and alternator fails. The PS-1290 I purchased for aux battery weights 6 pounds and has a rating of 9Ah which is more than I need. The one hour rating is 5.4 Ah which gives me over 1 hour of power for G3X, G5 and electronic ignition. I may switch to a smaller capacity and lighter battery as 45 minutes reserve electrical power should be enough. You can see in my schematic that I elected to use a single alternator. My reasoning is that a second alternator would weight just about as much but cost far more than the $30 it cost me for battery and diode. Wish I could remember who posted a similar design here that I copied and give him credit but I don't recall the name.
 

Attachments

  • Power.jpg
    Power.jpg
    124.5 KB · Views: 238
That's basically what I have in my -7A. Because of the diode, the Aux Battery doesn't see the full charge from the alternator. It sees about .7 volts less than the voltage supplied to the main battery. I use a small SLA $30 battery which seems to have more guts than the EarthX 12 I used for a while. I can live with the 3 lbs. heavier SLA battery versus the EarthX. I found I needed the Aux Battery to keep the G3X alive during the engine start sequence otherwise the G3X reboots just when you need your engine instruments the most.

I have a Slick magneto on the left and a Lightspeed Electronic ignition on the right. Lightspeed has you connect it's power directly to the main battery, in my case a PC680.
 
I did two pc680s, each with a contactor and a switch with three postitions - off, E-bus, on. The E-bus position feeds through a fusible link and then through the switch. I have two electronic ignitions. They feed directly from the two batteries through fusible links, pullable breakers, and switches. Normal batt switch position is both on for start and run. In case of alternator failure I would turn one battery off to hold it in reserve and the other to E-bus which doesn't include external lighting and pitot heat. When that runs low (10V) I'd turn the other battery to E-buss and start thinking about landing soon.

My load with alternator out and lighting off is just over 8A. At that draw one battery should be good for maybe an hour and a half. That's near 300 miles so I expect that I'll be able to find someplace where I can get an alternator overnighted to me and possibly even land at my intended destination. I could also run on one ignition at a time which would give me even better battery endurance and range if I thought it might be necessary.

This is all of course predicated on healthy batteries. The only way to know for sure is to load test them at the expected draw and see how long they hold up. West Mountain CBA works for that.

Ed Holyoke
 
I have a 7.2ah one for my EBuss, which weighs 4.3 lbs. I load test for 30 minutes once a year in actual use, driving everything I expect to use in IFR conditions for 30 minutes. I then replace it with a new one for less than $20 as part of the condition inspection, just because.
 
Thank you

Some really helpful stuff - thank you for all the responses. The pdf of the book is great too, that's a good find. Schematic diagrams are helping me get my head around it all as well.
Which wire do I connect in order to get smoke from my expensive avionics? I think I will skip that one.
Tim
 
You could create a switchable avionics bus, or just turn each one your avionics on and off manually before engine start and shut down.
 
Here's One Idea

This is what I'm using, mild variation on Nuckolls. The aux. battery is a 1.2ah gel cell. I'm 20hrs into phase 1 and I'm quite happy. Avionics are all wired directly but I'm likely to add a switch to the feed to the dual GRT Sports/ARINC adapter since if the ARINC adapter voltage has, albeit rarely, gone low enough during start so it stopped sending ARINC data to the EFIS. I just cycle all 3 buses and reboot everything but that's not optimum by any stretch. I may also just move the GRT ARINC adapter to the aux battery bus, will likely solve it an easier way.

Diagram is still a bit of a work in progress but is accurate but for the Emag/Pmag is now twin PMags and they are also wired to DB9s on the sub-panel for the programming lines. Their grounds go to the engine, NOT the firewall. The diodes are Shottky. The seat cigar jacks haven't been installed but are wired, they may eventually feed seat heaters/electric vests. So far no need but my wife will get a vote once she gets to ride.

I should mention my motivation for the 3 buses was ease of load shedding and ability to gracefully handle a dead alternator, dead main contactor, and/or dead main battery.

George
 

Attachments

  • Power Distribution Diagram.pdf
    85.5 KB · Views: 216
Last edited:
This is what I'm using, mild variation on Nuckolls.

A few comments. If it seems critical, I beg your pardon in advance. You've already installed yours, but others are looking.

Numbered diagram below.

1. Move the main bus feed to the alternator end of the 60 amp ANL. The ANL is there to protect against a short fed by battery discharge, the huge amp source, not alternator output. Yes, the pilot can open the main contactor manually in the vent of a short under the panel, but that takes time, the cabin is already filling with smoke, and the melting stuff damages other stuff. Better to immediately pop a big fuse. If you really want an ANL dedicated to the alternator line, fine, just install another ANL on the bus feed.

2. A pair of Schottkys? Diode drop is additive, so what voltage shows on the Mini, with the alternator online, and off?

3. Lot of stuff connected here, if this is really an endurance bus. If something is really needed, doesn't it belong on an essential bus? If we put all the stuff not really needed on the main bus, we can eliminate the endurance bus and one feed diode, which means the essential battery gets a higher state of charge. One can always fire up the main bus later in the flight, if desired.

4. Simple "short or open" what-if analysis goes a long way. For example, short either of these two wires to ground, and what happens? The diode burns open, and the panel goes dark. So you flip the E-Bus Alternate Feed switch...which feeds the short, and there is no fuse on that feed.

5. Odd duck here. The P-mags don't need battery support in flight, so why connect them to the last chance bus? All connections carry some risk. They belong on the main bus, for starting and ground idle.

6. As you noted, the best everyday use for an aux battery is to prevent EFIS brownout during start. Less recognized is that it would also prevent main EFIS reboot if main power is interrupted in flight, like when opening the main before closing the aux. It would not be flight critical, just disconcerting (here the GRT Mini would still be live), but it can be eliminated.

BTW, best if this feed wire is physically separated from the rest of the harness.
.
 

Attachments

  • Wiring.jpg
    Wiring.jpg
    104.3 KB · Views: 177
1. If the main bus feed is moved to the downstream side of the ANL-60 fuse, then if the alternator shorts out and blows that fuse, the main bus will lose all power. Bob Nuckolls does not fuse the main bus feeder on his electrical drawings.
.
#2. I agree with Dan. Two diodes should not be in series.
.
#3. Just call it the "E" bus. :)
.
#4. There is a 25 amp fuse protecting the E-Bus circuit.

#6. The aux battery could be used for brownout protection of the E-Bus.

#7. There is one too many buses. Simpler is better.
 
1. If the main bus feed is moved to the downstream side of the ANL-60 fuse, then if the alternator shorts out and blows that fuse, the main bus will lose all power.

Sure...but there is an alternate bus feed to cover that problem. Or just use the two ANL solution.

Bob Nuckolls does not fuse the main bus feeder on his electrical drawings.

Do you recall why?

#4. There is a 25 amp fuse protecting the E-Bus circuit.

Whoops, missed that, thanks Joe.
 
Thanks Dan!

I appreciate the feedback, thanks. A few answers to your questions:

1. ANL is to protect the battery in the event of an alternator short. Per Nuckols, fat wires to the buses don't get fused or fusible linked, he seems to think shorts on them are unlikely. You may be right.

2. Only time Aux Batt bus is getting juice through both diodes in series is when the Main is live and the E Bus is not. There is no flight regime/failure mode where that is planned. Main cascades to E cascades to Aux Batt via those two diodes. Main gets shut down until landing if battery goes south to load shed the main contactor. If the main contactor goes south, E Bus and Aux Batt bus still happy and I've still got most of my panel. Battery goes south then Aux Batt bus will get me down. At least that's the philosophy.

3. See above. Main Bus could the the All's Working, E Bus could be Contactor load shed bus, and Aux Batt Bus is aptly named. In any case, they all get thrown on at startup and off at shut down unless something goes sideways. I haven't recorded Mini voltage but it should be the main battery minus one diode worth of drop when all 3 are on, the same as the E Bus would be when the Main is on and the E Bus Alt feed is not, again, not a mode I see using.

4. E-Bus feed is fused 25a at the Batt Bus. Other fat bus-to-bus wires are not. See #1, you may be right.

5. Correct, they could. If I ever kill a battery though, hand-propping could be done with the Aux Batt Bus for the PMags. Not that I ever imagine hand-propping but there's no compelling reason to have them higher up the cascade, they are not a load shed item and their needs at highest draw are well within the Aux Batt Bus ability.

6. The loads on all the equipment on the Aux Batt Bus are pretty tiny. Again, it's the "dead battery" bus that gives me the Mini and EIS, transponder is on there since the Mini is the control head for it and they pretty well belong together. Minimal draw.

Again, no worries about criticism, I can learn from it. Just don't be upset if I don't end up compelled to change due to it ;-)

George



A few comments. If it seems critical, I beg your pardon in advance. You've already installed yours, but others are looking.

Numbered diagram below.

1. Move the main bus feed to the alternator end of the 60 amp ANL. The ANL is there to protect against a short fed by battery discharge, the huge amp source, not alternator output. Yes, the pilot can open the main contactor manually in the vent of a short under the panel, but that takes time, the cabin is already filling with smoke, and the melting stuff damages other stuff. Better to immediately pop a big fuse. If you really want an ANL dedicated to the alternator line, fine, just install another ANL on the bus feed.

2. A pair of Schottkys? Diode drop is additive, so what voltage shows on the Mini, with the alternator online, and off?

3. Lot of stuff connected here, if this is really an endurance bus. If something is really needed, doesn't it belong on an essential bus? If we put all the stuff not really needed on the main bus, we can eliminate the endurance bus and one feed diode, which means the essential battery gets a higher state of charge. One can always fire up the main bus later in the flight, if desired.

4. Simple "short or open" what-if analysis goes a long way. For example, short either of these two wires to ground, and what happens? The diode burns open, and the panel goes dark. So you flip the E-Bus Alternate Feed switch...which feeds the short, and there is no fuse on that feed.

5. Odd duck here. The P-mags don't need battery support in flight, so why connect them to the last chance bus? All connections carry some risk. They belong on the main bus, for starting and ground idle.

6. As you noted, the best everyday use for an aux battery is to prevent EFIS brownout during start. Less recognized is that it would also prevent main EFIS reboot if main power is interrupted in flight, like when opening the main before closing the aux. It would not be flight critical, just disconcerting (here the GRT Mini would still be live), but it can be eliminated.

BTW, best if this feed wire is physically separated from the rest of the harness.
.
 
Thanks!

Appreciate the feedback Joe, thanks!

1. You've covered it. It's been too long since I read it to remember his complete reasoning but it seemed Ok to me at the time. It is currently my mouse pad but I've got 90+ guests showing up at my house tonight in an hour or two so I'm going to reread it later.

2. See my answer to Dan, it mostly looks like two diodes in series, most of the time the EBus is juiced via the relay/Alt feed so it is seeing main battery voltage. Aux Batt Bus never gets main battery voltage but nearly always see only one diode worth of drop.

3. Sorry.

4. Correct.

6. Not as currently configured.

7. You may be right, De gustibus non est disputandum.

George


1. If the main bus feed is moved to the downstream side of the ANL-60 fuse, then if the alternator shorts out and blows that fuse, the main bus will lose all power. Bob Nuckolls does not fuse the main bus feeder on his electrical drawings.
.
#2. I agree with Dan. Two diodes should not be in series.
.
#3. Just call it the "E" bus. :)
.
#4. There is a 25 amp fuse protecting the E-Bus circuit.

#6. The aux battery could be used for brownout protection of the E-Bus.

#7. There is one too many buses. Simpler is better.
 
Dan asked why Bob Nuckolls does not protect the main bus feeder with a fuse.
His reasoning can be found at the end of this thread:
https://vansairforce.net/community/showthread.php?p=335866
I paraphrased his posts below:
Most type certificated small planes do not fuse the main bus feeder.
Quote from FARs:
If the ability to reset a circuit breaker or replace a fuse is
essential to safety in flight, that circuit breaker or fuse must be so
located and identified that it can be readily reset or replaced in flight.
.
Losing the main bus power could be considered a safety of flight issue.
How can the pilot replace an ANL current limiter while flying?
(Some FARs do not apply to experimental aircraft.)
.
When a fat copper cable shorts to aluminum, the aluminum will be burned away. The copper will
remain intact. Faulted robust wires generally arc to ground and burn their faults clear.
.
If there is smoke, the pilot can shut of the battery contactor and alternator.
 
2. Only time Aux Batt bus is getting juice through both diodes in series is when the Main is live and the E Bus is not. There is no flight regime/failure mode where that is planned.....they all get thrown on at startup and off at shut down unless something goes sideways.

Ahhh. This is new to me. My understanding of the Nuckolls Alternate Feed is what the name implies, an alternate supply to be switched on if the pilot deems it necessary to open the master contactor. I've never considered the idea of operating with the Alt Feed live all the time. Is it something endorsed in AEC forum discussion?

Recall the Gomez story leading AEC Chapter 17? Pilot Gomez twice suffered a shower of sparks from under the panel. Nuckolls then builds a case for the substitution of a diode-supplied essential bus, with an alternate feed switch (pg 17-7). How to use it is found a few pages later, under the heading of "Putting it all Together":

Turn the essential bus alternate feed switch ON, battery/alternator switch OFF and continue flying the airplane to a comfortable arrival.

I will again point out that a hard short on either of the two wires marked #4 has the potential to take down the endurance bus, so they need some care. And the alt feed (here an unprotected #6 or #8?) should be routed separate from the main bus feed, in particular when melting airframe parts are considered acceptable...

Dan asked why Bob Nuckolls does not protect the main bus feeder with a fuse.
His reasoning can be found at the end of this thread:
https://vansairforce.net/community/showthread.php?p=335866

I remember now, the "structure as a fuse" theory. The key argument seems to be the aluminum structure will melt away and disconnect the short before an ANL could open, thus rendering the ANL useless ballast. May be valid for aluminum sheet metal, but the potential sites include (for example) the stainless steel firewall, the steel pass-thru fitting, or steel bolts. Personally I like my melting inside a fuse capsule.

Again, given the inclusion of an alt feed, there's not much downside to fusing the main bus feed. Heck, here I'd ANL the alt feed too. Both avoid the irony of justifying the alt feed concept with sparks and smoke (Mr. Gomez again), while advocating sparks and smoke as a control method.
 
Last edited:
Agreed, the firewall is the most likely place for the main bus feeder to short to ground. A bushed (a.k.a. chase) nipple lined with firesleeve will minimize the danger.
Charlie, a frequent and very knowledgeable poster on the AeroElectric list elected to use fusible links instead of ANL current limiters. They are the blue wires in this picture.
 
1. ANL is to protect the battery in the event of an alternator short.

end quote

If the alternator shorts, all the current available in the battery will flow through the B-lead to the short. The ANL is to protect that wire not the battery or, for that matter the alternator which has just failed anyway.

If the alternator runs away and produces too high a voltage, the ANL won't do a thing to stop it. That's what the crowbar is for. It shorts the alternator field to ground taking the alternator offline. Without the crowbar the avionics and, sooner or later, the battery would also be damaged.

Ed Holyoke
 
Back
Top