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Dual Batteries or Dual Alternators

wilddog

Well Known Member
For an airplane with all electric fuel pumps and ignitions, what are the pros and cons of each option?
 
It is best to isolate dual batteries (two bus system). This is essentially two seperate systems. The cons - expensive, more complicated, takes longer, double the weight as well. Dual batteries alone - double the available electrons, added weight. If you go dual batteries, you might as well do dual alternators as well.

I used one battery, dual alternators on my RV-10. All glass system. No problems todate.
 
There is nothing more reliable than a battery (assuming you have not abused it).

Two PC-625 provide ample cranking power. With single alternator and two master solenoids you run them in parallel for normal operations. If the alternator fails, you open both masters to split the panel loads between each battery (left side from left battery, right side from right battery, each connected to the battery via a 30 amp relay).

Two PC-625 batteries will provide 34 amp hours of reserve power. For practical terms, this is about two hours for most glass panels - assuming you don't have the landing lights, pitot heat and such on. On my RV-8A I mounted one battery on each side on the cockpit floor right behind the firewall. W&B worked well. On the RV-10 both batteries are mounted in the normal spot aft of the baggage compartment.

If you add a standby alternator, take the output via two 40 amp diodes and have one connection to left, one to right panel busses. Keep the rest rest the same. This way you can isolate most common fault modes to minimize the risk of loss of power to your vital loads.

Carl
 
Single battery, dual alternators here. My backup alternator is a B&C 20amp on the vacuum pad, it can handle all flight-critical loads.
 
Dual battery

and dual alternators for me. Installed cross tie solenoid. Accepted the weight to insure safe operation of a electrically dependent system.
 
It is extremely unlikely that a battery will develop a fault mid-flight. The more common scenario is the alternator goes bad and develops an over-voltage condition or simply stops charging the battery. A small backup alternator on the engine accessory case corrects this condition and provides for safe continuation of flight to your destination, assuming your are not exceeding the amperage rating of your backup alternator. Careful consideration of your continuous use, 'must have' avionics and their collective electrical load and use of an e-bus for those items will take the stress out of such an event.

Check out Bob Knuckoll's Aero-electric Connection for the details.

erich
 
+ 1 for what longline said. Secondary system is 8a alternator and small Life battery with crosstie. Came in handy when I accidently left main master on and discharged main battery.
 
and dual alternators for me. Installed cross tie solenoid. Accepted the weight to insure safe operation of a electrically dependent system.

+ 1 for what longline said. Secondary system is 8a alternator and small Life battery with crosstie. Came in handy when I accidently left main master on and discharged main battery.

Yep, same here. Dual batt and alt. And cross tie for extra start boost.

Aux alternator and battery dedicated to the e buss and a Schottky diode allows main batt to power e buss if aux battery goes down.
 
Dual batteries, single alternator. One PC-680 for turning starter, and a little lightweight one to drive one of the CDIs.

VFR Day-Night Ops.
 
Dual Everything

Dual Alternators (B&C 40-amp Alternator and SD-8-amp Dynamo); Dual Batteries (EarthX ETX680C's); all through an EFII Bus Manager here.
 
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I think it depends on the mission.

IFR? Dual batteries, dual alternators, dual regulators.

VFR? Big battery, schottky-fed small battery for second ignition, single alternator...KIS.
 
electrical system evolution

Dan's got it right,
Electrical system complexity follows a progression:
1. One batt, one alternator
2. Two batts, one alternator
3. Two batts, two alternators

One batt, two alternators is a problem. If you lose the batt, all you have left is half wave rectified AC power off the alternator - this means unhappy electronics. There needs to be a functioning battery in the system to absorb the pulses from the alternator.

Robert
 
60 amp main alternator, SD8 standby alternator on vacuum pad, one battery. Split Main bus and Emergency bus. Aeroelectric book has the setup in one of their diagrams. No time limit this way as with a battery.
 
One batt, two alternators is a problem. If you lose the batt, all you have left is half wave rectified AC power off the alternator - this means unhappy electronics. There needs to be a functioning battery in the system to absorb the pulses from the alternator.

Im not enough of an expert to argue with you here, but I consider Bob Knuckolls to be, and he would disagree with what you say above. I tend to respect his opinion.

erich
 
batts and alts

There really isn't anything to argue, that's how alternators work.
An important part of the evolving aircraft electronics systems is the evolving electrical distribution to support the new systems.

Robert
 
Alternator Output

Hi Erich,

It?s Bob Nuckolls (spelling). Through Aeroelectric Connection and his various articles, Bob has contributed a tremendous amount to experimental aviation. I?m somewhat following Bob?s z14j architecture myself substituting an SD-8 for the auxiliary 20-amp alternator and inserting the EFII Bus Manager into that architecture. Bob is definitely one of our ?good guys!?

That being said, just because Bob offers a schematic, that doesn?t mean that there might not be component issues associated with the schematic that you need to research and understand yourself. Yes, the electrons will keep flowing from the alternators if the battery fails, but, as Robert correctly mentions, the alternators in absence of a battery deliver a wave form that some electronics like avionics or ignitions may not like. Yes, most will keep operating, but possibly with signal (noise) problems or other issues. Components like lights or fuel pumps are usually not as sensitive.

I should mention that Robert Paisley is an Electrical Engineer (Cal Poly) and has decades of experience designing, producing, and testing fuel and ignition systems for motorcycles, cars, and, over the past decade, aircraft. He definitely knows what he is talking about and is one of our ?good guys,? too. Rather than posting a ?put down? here on VAF requiring a response or pitting one experimental aviation ?good guy? against another, it might be best to call or send a PM or email first. Assuming that Bob would disagree with Robert is a real stretch and is probably not the case.

Robert?s (EFII?s) phone is 951-317-3473, you can PM him here on VAF via rcpaisley, and EFII?s email is [email protected].

I hope this helps,
 
There really isn't anything to argue, that's how alternators work.
An important part of the evolving aircraft electronics systems is the evolving electrical distribution to support the new systems.

Robert

All true, however can you please expand on "unhappy electronics"?

I've tested this very situation in my aircraft with dual alternators and a single battery. I took the battery offline by switching off the master contactor, to test whether the alternator(s) would continue providing power. The result was, if the main alternator field is already energized when the battery goes down, then it continues providing power. I even tried turning on and off various loads (pitot heat, landing lights, etc.) to test whether I could trick the alternator regulator (B&C) into going offline. Nope, still worked. Take the main alternator offline in addition to the battery failure however, and there's nothing left to energize the standby alternator field. (But now we're talking simultaneous, independent failures which is another debate altogether.)

Yes, you could hear the AC ripple noise through the intercom, but the electronics (full Skyview suite and Garmin IFR GPS navigator) kept happily humming along and sustained no temporary nor permanent damage.

edit: I do not have electronic ignition. Robert obviously knows more about his gear than anyone else, so if he says AC ripple is bad for his system then you can take that to the bank! :)
 
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I want to be able to continue a flight with no disruption and without alarming my wife if the main alt fails so I have a BC 20a backup with a single main battery. I also have a small EarthX for an avionics back up which can be crosstied to the main bus if needed. In addition I also added a couple of large capacitors on the main and avionics bus in case the battery relay fails there will still be some AC filtering to keep the avionics happy.

I would also have no hesitation about departing with only the backup alternator operating (VFR). There will be no side trips to Autozone or O'Reilly's for me.

My wife (like most) doesn't want any excitement in the cockpit, if the main alt failed she would have no idea it happened and I probably wouldn't tell her until we got to our destination :eek:
 
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robust electrical

There is always more than one way to solve any electrical requirement.
We use a very simple and robust scheme that doesn't include a master relay, or a typical aircraft right/left/both type switch. We also connect alternator outputs directly to batteries to avoid load dump. This is when an alternator gets separated from its battery and dumps its AC output through the rest of the systems.

Drawing 5A in our Bus Manager instructions (http://www.flyefii.com/media/Bus_Manager_Installation_Instructions.pdf) shows how simple and robust, redundant electrical supply can be.

Robert
 
Alts & Batts

I believe all Robert is saying is that it is always preferable to have a good battery online from an electronics standpoint.

I personally think it?s a great idea to test dual-alternator, single battery systems to see what happens when the battery is removed from a running system. On the other hand, before testing, I would certainly want to research and understand what will probably happen first. Prior analysis and data predictions are good! Smoke is bad!

I also think it?s a great idea (Walt!) to have a small backup battery lurking in the background which the pilot can switch-in, if needed, to power electronics and also help damp the alternator output (. . . essentially, another form of dual-alternator, dual-battery). The capacitors are a great move, too. Are those ?flux capacitors,? Walt?! ;) (re: ?Back to the Future?)

What is still unclear to me, and maybe someone here can help me with this, is how the B&C linear regulator might, or might not, help mitigate alternator-only output. I have a corresponding email into B&C on this, but does anyone know the answer up-front? From the B&C literature, it is clear that the regulator itself does not generate any noise, but it also appears that noise problems must be discovered and solved at the source rather than via the regulator, including switching off the alternator(s) as a test.

Thanks,
 
I've tested this very situation in my aircraft with dual alternators and a single battery. I took the battery offline by switching off the master contactor, to test whether the alternator(s) would continue providing power. The result was, if the main alternator field is already energized when the battery goes down, then it continues providing power.

Same test here, with a single Plane Power. Initially I had the same result, but it dropped off line a short time later.

Recall I've also reported a charging failure due to a poor field plug connection. Perhaps dropping off line and the later plug failure are related. With both internal and external regulators, self-excitement of an alternator set up for aircraft requires a current path from B-lead to main bus, then back to the field terminal, via terminals, crimps, breakers, and switches. Without a battery feed, just the slightest momentary interruption anywhere in the circuit causes the alternator to quit.

Yes, alternators should maintain output without an external battery, but maybe it's not a bulletproof assumption.
 
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I dont get it

Can someone explain to me why you would be flying along, your battery fails (not exactly sure what everyone is defining as a flying battery failure in the first place), and you switch it out of the circuit?

IOW, why would you take the batt off line?

If its gone flat, its still acting as a capacitor and smoothing any ac ripple.

If you got a fire, then all your electricity gets shut down, and your motor flies on pmag or magneto.

What am I missing in the discussion here?
 
If single batt and alt, and from vibration or what ever, a battery plate opens, then you are reliant on the alternator to keep the power flowing, if for a second in this condition, there is any disruption in power to the field, the power flow stops and that is bad for an electric dependent aircraft, I will have main, backup and Dynon back up batteries. (4 total)
 
I've actually built airplanes with both electrical systems. Rocket one was one battery with dual alternators. Rocket two was two batteries with one alternator.

I used a battery management device used in the boating industry to handle the charging.

My vote is two batteries and one alternator. Less components, less cost, less complexity, and more reliability. It's been said already, nothing more reliable than a well maintained battery, and more than enough power to make an alternator failure a non-event.

Just my $.02.
 
If you got a fire, then all your electricity gets shut down, and your motor flies on pmag or magneto.

What am I missing in the discussion here?

The condition stated in the original question...with all electric fuel pumps and ignitions. The ability to open the master contactor without affecting engine operation is a key system test.

The KIS approach to that condition is to wire the ignitions and pumps directly to battery feeds which have no dependence on the aircraft electrical system.

Basic VFR and IFR architecture:



I'll submit three principles, often overlooked while designing grand electrical schemes with many wonderful features.

1. Any pilot should be able to operate it without instruction. Put another way, operation should be self-evident, or the same as any other airplane.

2. The battery feeds should be physically separated from the rest of the wiring, to the maximum extent possible. They should also be separated from each other.

3. The least complex system is usually the most reliable.

Humor a bit of editorializing.

Right now, we're growing a fleet with no electrical commonality. At the same time, we're bringing in more and more non-builder pilots from the traditional GA fleet. E-busses, cross feeds, and switch flipping to maintain engine noise is a recipe for increased accident numbers. Time and time again, the human factor is the stubborn cause we can't make go away.

Physical separation is important. We see a nice wiring diagram intended to isolate problems, but then the builder installs the backup alongside the primary. Doing so makes them subject to the same physical trauma, whatever it might be, and one can take out the other.

As for the least complex being the most reliable, let's go with an analogy. You've made a successful forced landing in a forest clearing 100 miles from civilization, and baby, it's cold outside. The chips are down; without fire and shelter, you're going to freeze to death. Which would you rather have, a chain saw, or an ax?
 
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The condition stated in the original question...with all electric fuel pumps and ignitions. The ability to open the master contactor without affecting engine operation is a key system test.

The KIS approach to that condition is to wire the ignitions and pumps directly to battery feeds which have no dependence on the aircraft electrical system.

Basic VFR and IFR architecture:



I'll submit three principles, often overlooked while designing grand electrical schemes with many wonderful features.

1. Any pilot should be able to operate it without instruction. Put another way, operation should be self-evident, or the same as any other airplane.

2. The battery feeds should be physically separated from the rest of the wiring, to the maximum extent possible. They should also be separated from each other.

3. The least complex system is usually the most reliable.

Humor a bit of editorializing.

Right now, we're growing a fleet with no electrical commonality. At the same time, we're bringing in more and more non-builder pilots from the traditional GA fleet. E-busses, cross feeds, and switch flipping to maintain engine noise is a recipe for increased accident numbers. Time and time again, the human factor is the stubborn cause we can't make go away.

Physical separation is important. We see a nice wiring diagram intended to isolate problems, but then the builder installs the backup alongside the primary. Doing so makes them subject to the same physical trauma, whatever it might be, and one can take out the other.

As for the least complex being the most reliable, let's go with an analogy. You've made a successful forced landing in a forest clearing 100 miles from civilization, and baby, it's cold outside. The chips are down; without fire and shelter, you're going to freeze to death. Which would you rather have, a chain saw, or an ax?

Well put. I use the VFR architecture in my Rocket. If I want to fly IFR in IMC, I would use a modified version of the IFR architecture with the two alternators, each driven from their own engine!
 
alternator and generator

I have a 60 amp alternator with a 10 amp generator and single battery. The generator fuse is pulled and only pushed in if the alternator fails. It will run one gps/com, transponder and LED nav lights. I have a back up battery on my dynon and aera 560. My son designed the system and I haven't checked it yet, but according to an electrician that installed the generator it does work.
 
For an airplane with all electric fuel pumps and ignitions, what are the pros and cons of each option?

What ignition system are you installing? If you are going to use P-mags, then there is no electrical load, other thsn for starting.

The rest of your electrical system only needs to keep your fuel pump(s), Nav gear and EFIS alive. There is no need for a radio, transponder, etc. as there are standard procedures for dealing with those outages.
 
60 amp main alternator, SD8 standby alternator on vacuum pad, one battery. Split Main bus and Emergency bus. Aeroelectric book has the setup in one of their diagrams. No time limit this way as with a battery.

Hello my friends, does anybody have the diagram and or pictures of how and where install each component of the diagram?? And a picture of the switches into the cockpit...
I am installing in my RV-7 the dual alternator, single battery shown in Aeroelectric book. I bought all the list of materials in B&C, and I?m installing the full FlyEFII system.
It will help a lot if anybody shows a picture of the installation! Thanks!
 
Hi Erich,

It?s Bob Nuckolls (spelling). Through Aeroelectric Connection and his various articles, Bob has contributed a tremendous amount to experimental aviation. I?m somewhat following Bob?s z14j architecture myself substituting an SD-8 for the auxiliary 20-amp alternator and inserting the EFII Bus Manager into that architecture. Bob is definitely one of our ?good guys!?

That being said, just because Bob offers a schematic, that doesn?t mean that there might not be component issues associated with the schematic that you need to research and understand yourself. Yes, the electrons will keep flowing from the alternators if the battery fails, but, as Robert correctly mentions, the alternators in absence of a battery deliver a wave form that some electronics like avionics or ignitions may not like. Yes, most will keep operating, but possibly with signal (noise) problems or other issues. Components like lights or fuel pumps are usually not as sensitive.

I should mention that Robert Paisley is an Electrical Engineer (Cal Poly) and has decades of experience designing, producing, and testing fuel and ignition systems for motorcycles, cars, and, over the past decade, aircraft. He definitely knows what he is talking about and is one of our ?good guys,? too. Rather than posting a ?put down? here on VAF requiring a response or pitting one experimental aviation ?good guy? against another, it might be best to call or send a PM or email first. Assuming that Bob would disagree with Robert is a real stretch and is probably not the case.

Robert?s (EFII?s) phone is 951-317-3473, you can PM him here on VAF via rcpaisley, and EFII?s email is [email protected].

I hope this helps,

Hi my friend, do you have the diagram of how to insert the bus manager into the dual alt single battery architecture?
 
Single main battery, dual alternators, EFIS backup battery

I have a Plane Power 60A belt driven alternator that runs at 14.4 volts, and now the Plane Power 30A vacuum pad driven backup alternator that runs at 13.6 volts. The backup alternator does nothing while the primary alternator is supplying power. If the primary would shut down, then the backup starts to make current. I can test the backup by opening the primary field breaker and then see the voltage drop to 13.6 and verify charging current from the backup alternator. I use an AGM Harley Davidson motorcycle battery which I just replaced after four years of faithful service. I also have a backup battery just for the pilot side Dynon EFIS. This is nice for starting the engine by booting the EFIS from the backup battery prior to turning on the Avionics Bus.
 
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