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Indulge me...

Bill Boyd

Well Known Member
I'm inching closer to the point of having to commit to a power distribution architecture for my 10, and there are some fundamental considerations I have yet to resolve. I shall tap the Hive Mind and all will be good :D

My thoughts going in: some level of redundancy is essential for safety of flight in an electrically-dependent aircraft especially for electrically-dependent (no-vacuum) IFR flight. While the basic combination of alternator and battery offers some margin based on battery charge and health, this is too bare-boned to be prudent. Remaining energy in the battery at the time the light comes on may not be near enough and can never be precisely known as the battery ages, and in the event of a battery failure, the alternator may not behave properly to deliver steady, regulated voltage to the bus.

A dual-alternator, dual-battery "Cadillac/NASA" system offers very comfortable margins - but at considerable cost and weight penalties, from my personal perspective.

By process of elimination, the sweet spot is a dual-alternator or dual-battery architecture. The question is, which one is preferred, and why.

I'd really like to hear some thoughts on the relative merits of doubling down on alternators vs. batteries, particularly as it pertains to RV-10 W&B and installation challenges. Thanks!

(And if this has been hashed before, kindly redirect me to the thread.)
 
Options

Are you going to be using electronic ignition/fuel injection? It changes the requirements if you are...and there have been quite a few debates about "what is best".

See this one: http://www.vansairforce.com/community/showthread.php?t=151435&page=19

That being said, it all comes down to what level of risk you are comfortable with; you CANNOT design a perfect system so you need to be comfortable with what happens if things fail.

I opted for a system based on the Z-14 detailed in The AeroElectric Connection. It it slightly different, having taken advice from several folks flying already with similar aircraft to my own.

I am running EFII, so my system ended up being a dual alternator, dual battery, split bus, with an essential engine bus that is powered from both systems...
 
During the panel upgrade, I chose to go with dual alternator (B&C pad mounted backup) and a single battery (EarthX) firewall mounted.

I also have a G5 with backup battery.

I feel that in the -10 this worked best for me, as keeping as much weight forward is advantageous in terms of loading passengers and baggage. My CG is right at the forward limit with pilot and fuel, so I can load the seats and baggage area easily.

I felt like the B&C stuff is very reliable, and this is a fairly hands off / maintenance free solution to electrons - no extra battery to maintain / check / replace.
 
Yep - it has a been hashed several times. I offer:
- A well maintain battery is the most reliable single component in your airplane. Getting power out of it when you need it however can be problematic and should be your focus. I replace one battery every three years or on any occasion that I think it has been abused (e.g. master left on). This provides some assurance on battery health.
- Taking the above a step further, a fault that takes down power from your panel is most likely not from a failed battery. As such, here is where a design that directly examines all the relays, switches, connections and such that can fail leaving you with a dark panel pays off. This simple example is your single avionics relay fails.
- The impact from loosing the single alternator as the sole design focus, in my opinion, turns a blind eye to most “dark panel” lessons learned.
- I have no faith in any alternator running without a battery. I’ll let other spin what they will on this but for my airplanes no way, no how will I design that into the electrical system.

Bottom line:
- Two alternator, single battery schemes are only marginally better than single alternator, single battery. The only risk mitigated is the loss of the primary alternator. This is not the leading reason for a failure giving you a dark panel.
- If you jump to a two battery, single alternator foundation you can, with some thought, build a very robust power distribution that will have two hours or so of reserve capacity on the loss of the single alternator.
- I use two PC-625 batteries, both on line for starting and normal operation, split out (opening the two master solenoids) as the first immediate action on any electrical fault to establish the most reliable configuration. One battery feeds Avionics #1, the other Avionics #2 but either buss can be selected to either battery. The objective is no single fault, without pilot action, will take down more than half of the panel (so one EFIS, one GPS and one COMM are always up). After the pilot examines the situation the other avionics buss can be restored by single pilot action.

I did backfit a 20 amp standby alternator on the RV-10 in addition to two batteries as I wanted unlimited electrical reserve (as in full IFR flight to fuel exhaustion instead of two hours, or if need be a way to get home after stopping for gas). On reflection I decided to not install the second alternator on the new RV-8.

One last consideration. I offer a thoughtful dual PC-625 (or similar) install eliminates the need for the plethora of EFIS backup batteries that now seem to be in fashion. My point - there is little weight penalty.

Carl
 
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I went with dual alternators and single battery, with the EarthX ETX900 which is arguably oversized for my airplane but that was kind of the point. My Dynon screens have their own battery backup as well which is tested annually to show at least 45 minutes reserve or they get replaced.

Single alternator failure for me means continued flight on the backup, and replacement before further flight - it allows me to complete the in-progress mission as planned, but not launch into another.

Dual alternator failure in a single flight is highly unlikely - and I treat it the same way I would treat an electrical fault behind the panel that suddenly started dumping smoke into the cockpit - hit the master/alt switch and make everything cold is step #1. Now - in my airplane that means the big fan up front stops making noise - so my step #2 consists of bringing up my dedicated backup power (wired direct from the battery prior to the master) to my electric fuel pumps, and with one standard mag on the engine I have power again. The ETX-900 will pull that single fuel pump for almost 2 hours, and it's the only load I have on the battery wired direct. The Dynon screen gives me a minimum of 45 minutes IMC capability to either get down or troubleshoot my electrical problem to bring at least one alternator and partial panel back online. In VFR conditions I can shut down the Dynon to save its internal battery power while I do a longer troubleshoot if necessary, and bring it back up later.

The scenario of catastrophic battery failure or BMS disconnect or master contactor failure is handled by the alternator continuing to supply power to the panel without the battery online - I tested this shortly after Phase I, accidentally as it happened, but it worked just fine. I've got enough electrical load in my airplane that the alternator maintained a pretty stable voltage (it hunted up and down about a quarter volt each way) with the battery offline and kept it all going until I figured out that I had bumped my master switch accidentally. Later I tested it again (intentionally this time) under more controlled conditions and got the same result, so I'm happy with it.

My only serious exposure to electrical failure is an IFR approach with dual alternator failure (or electrical fire/smoke), running on battery only for the fuel pump. I've got my Dynon screen but not my Garmin 430W or autopilot, so in that case I would have to complete a (non-legal but arguably safe, at least safer than the alternative) GPS approach somewhere by following my approach plate on the Dynon which will geo-reference my aircraft over the plate, and hand-fly it with altitude drop-downs. If the 430W is not the device which continues to emit smoke during troubleshooting, then I can bring it back online and fly a legal GPS/ILS approach anywhere at the expense of shortened runtime on my battery.

You can't build a completely fault-tolerant electrical system in any airplane. There will always be some level of risk that has to be "acceptable" and that risk level is highly individualized. You should think about the likelihood of failure for various components, build in backups for the ones you consider critical, and accept the potential failure of the rest.
 
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In my system I considered reliability/redundancy vs. weight/cost/safety. The common IBBS EFIS backup battery is a Li-Fe-Po4 chemistry, weighs 2 pounds and costs about $400. A PC680 is a lead/acid AGM chemistry, weighs 15 pounds and costs about $130. The IBBS must be mounted in the cabin, the PC680 can be firewall or aft fuselage. Both require periodic maintenance and replacement (IBBS requires annual load testing for continued airworthiness, and replacement following a failed load test). For my safety I decided not to have a Li-Fe-PO4 battery in the cabin. The only battery backup is the Garmin supplied one for the G5 backup EFIS.

For IFR I decided on the Z-14 dual battery/dual alternator architecture. The backup alternator weighs about the same as the obsolete vacuum pump/vacuum regulator/hoses/vacuum instruments that are replaced by the EFIS. My penalty is (15 lbs - 2 lbs)=13 lbs. On the plus side is a safer and more cost effective battery and battery installation.
 
To re-state what might be obvious, dual bat is cheaper but heavier, and duration-limited by load an bat size & health (a potential big variable).

Dual alt will be more expensive (with a pad mount alt2) and lighter, with the advantage of unlimited duration.

And contrary to Carl's belief ( ;-) ), it's not hard to avoid single-point failure with a 2-alt, single bat system.

With most alternative engines, dual alt is both lighter *and* cheaper.



Charlie
 
I don't think your electrical system would be very happy being powered from your alternator without a battery in the loop.
 
To re-state what might be obvious, dual bat is cheaper but heavier, and duration-limited by load an bat size & health (a potential big variable).

Dual alt will be more expensive (with a pad mount alt2) and lighter, with the advantage of unlimited duration.

And contrary to Carl's belief ( ;-) ), it's not hard to avoid single-point failure with a 2-alt, single bat system.

With most alternative engines, dual alt is both lighter *and* cheaper.


Charlie

A thought or two: dual alt becomes less-more-expensive :) if alternators practically never need replacing as they age, but batteries do...

and dual battery isn't necessarily heavier if the batteries under discussion are LiFePO4.

I'm planning IFR but Bendix fuel injection, so motor will run without ship's power. At the moment, given a 10+ year ownership time frame, dual alternator with an EarthX 700 in the standard location is looking like the best intersection of value, weight, reliability.
 
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But dual lithiums will buy a *lot* of alternator. :)

Not needing electrons for the motor makes the task much easier.
 
SNIP

And contrary to Carl's belief ( ;-) ), it's not hard to avoid single-point failure with a 2-alt, single bat system. SNIP

Charlie

Please share how you do this as I have never seen a single battery IFR set up that does not have such weaknesses.

Carl
 
I don't think your electrical system would be very happy being powered from your alternator without a battery in the loop.

Oh agreed - but if it's a choice of causing temporary pain to the panel, versus very real and permanent pain for me in the form of cracking up the airplane - guess who wins? I can build another panel. We're talking about emergency operations here - equipment lifetimes of double-digit minutes are plenty acceptable if it gets me on the ground in one piece.
 
Please share how you do this as I have never seen a single battery IFR set up that does not have such weaknesses.

Carl

I'm on the road right now but pm me your email and when I get home, I'll send you a copy to critique. I no longer waste time linking pics here. :)
 
Please share how you do this as I have never seen a single battery IFR set up that does not have such weaknesses.

Carl

I think by definition, if you have only one battery, you have a single point of failure. It may be a very unlikely one, but it is a single point none the less. There does seem to be varying opinion on whether the alternator(s) will continue to operate properly in the absence of a battery. I've done it a fair bit on old cars with no issues, but I don't really know. However, absence may not be the problem.

If I had a single battery in such a system, it would NOT be a Li-Fe-PO4 battery.
 
Steve,

Then you've not 'don't think(ed)' ; you've done. As in demonstrated that no-battery alternator operation works. Now, as an engineer, you can show the math on exactly how much ripple (variation above/below reference) is in a 3 phase AC voltage after being rectified to 14V DC.

Those of you who are subscribed to the Aeroelectric Connection have already seen the answer. :)
 
Steve,

Then you've not 'don't think(ed)' ; you've done. As in demonstrated that no-battery alternator operation works. Now, as an engineer, you can show the math on exactly how much ripple (variation above/below reference) is in a 3 phase AC voltage after being rectified to 14V DC.

Those of you who are subscribed to the Aeroelectric Connection have already seen the answer. :)

And I tested it in flight. Intentionally. Ill thought out? Careless? Reckless? Pick your adjective, and I will probably not argue with you, but it worked. About 7 or 8 minutes, if memory serves. I wanted to know. Data trumps opinion every time.
 
I'm in the dual alt w/single bat camp. In case the 'Gigavac' contactor falls off line the alt's will continue to function normally. I added a couple of caps to the main and avionics buses which will help in the event the battery disconnects and also help to keep a nice smooth DC on the buses at all times.
(this is old picture and some things have changed but the caps are still there)
Batteries rarely fail, never used the back-up alt except to test occasionally, nice to know its there in the event the main alt fails.
No LiPo either, would love to save the weight but not worth the risk to me personally.

006-L.jpg
 
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That is how it's done!

"...No LiPo either, would love to save the weight but not worth the risk to me personally..."

Nicely stated!

It is too bad that there are folks who would make this simple statement into a discussion about how you will crash and burn in a fiery hole if you use a lithium battery...just because THEY don't want to use that particular technology.
 
I'm in the dual alt w/single bat camp. In case the 'Gigavac' contactor falls off line the alt's will continue to function normally. I added a couple of caps to the main and avionics buses which will help in the event.....

006-L.jpg

In my experience, as an electronics technician, and recently in three different residential systems, Caps are the most unreliable component in most electrical systems. When they fail, they typically fail as a partial to dead short.
Most modern avionics can handle dirty power. I would rather have a dirty bus than a potential dead short.
Caps don?t age well either. If I had them, I would change them out every few years.
Of course, we can chase possible points of failure all day long, so this is just my opinion for what it?s worth.... there are a lot of Caps used in our airplanes - anything with its own power supply or associated filter. However, they are component specific, and won?t take your bus down.

One more thing to think about.
 
In my experience, as an electronics technician, and recently in three different residential systems, Caps are the most unreliable component in most electrical systems. When they fail, they typically fail as a partial to dead short.
Most modern avionics can handle dirty power. I would rather have a dirty bus than a potential dead short.
Caps don?t age well either. If I had them, I would change them out every few years.
Of course, we can chase possible points of failure all day long, so this is just my opinion for what it?s worth.... there are a lot of Caps used in our airplanes - anything with its own power supply or associated filter. However, they are component specific, and won?t take your bus down.

One more thing to think about.

Modern 'high quality' electrolytics properly rated and used as simple DC power filters are quite reliable. Many of the cheap Chinese made units used in switching power supplies these days do suffer from premature failure. Not really the same application, but I will agree that every component adds some level of failure mode and should be carefully evaluated.
 
Bill,

Based on your mission being IFR, I think you'd do well with a single-battery, dual alternator system. You would also have additional small battery(s) to power individual components. These days, many backup EFIS displays come with a built-in backup battery to provide the backup horizon and other instruments needed for IFR flight. Many primary EFIS also have a backup battery capability (just for the EFIS, not the entire bus). And then to round it out, I'd recommend a handheld radio with a headphone jack adapter for backup comms.

So effectively this is a dual battery system, but different than the older traditional dual bus/dual battery architecture and more appropriate for homebuilt aircraft IMO.

With LED lighting drawing so little current and therefore low current draw overall, you can easily fly home VFR on your backup alternator.

My 2 cents... :)
 
The argument for twin batteries-

I think a strong argument can be made for dual batteries, especially lightweight lithium batteries, mainly for one reason. If you have essential electrical loads like electronic ignition or primary electric fuel pumps, or even an auxiliary partial avionics bus, switches for these loads should not be connected in any way to the main bus, but instead to an always hot, hard-connected-to-the-battery essential loads bus. With a symmetrical-twin two-battery system like the one I?m now installing, EACH battery has an essential loads bus, and each essential load can be powered by an on-off-on switch that can be connected to EITHER battery with impunity. This is a beautiful form of redundancy. Each battery has its own contactor to the main bus and the master switch is also on-off-on so either battery can, at any moment, be serving as primary, and the other as secondary.
2v2EASpGGxBELK5.jpg

As you can see in this mockup of the graphics for my new panel, there can also be a momentary two-pole switch that, when held in the closed position, allows the engine to be started using both batteries, or allows the batteries roles to be switched in flight without interrupting power to the avionics, alternator field, or other sensitive electronics. It is virtually impossible to inadvertantly discharge both batteries after an undetected alternator failure.

For a detailed description of my system, see post 39 in this thread, reading the quote first:
http://www.vansairforce.com/community/showthread.php?t=169270&page=4
 
Great thread here with inputs from those I consider experts on this subject. I'm in the middle of 'thinking'.... AKA waiting on re$ourse$.... of changing my 7A to an IFR airplane. Been reviewing/reading lots of threads on the subject. I've received and reviewed Carl's electrical schematics and find myself in his camp. I will have dual batts with a redundant way to deliver that power to those critical instruments in the event of alt failure. My engine doesn't require an outside source of electrons to remain happy. My goal would be to get back into VMC weather or back on the ground ASAP in the event of an alt failure.

It would be interesting to know what the failure statistical numbers would be regarding the setups of dual alt/dual batts..... dual alt only..... dual batts only.
I know I'm talking from a very inexperienced position here but it seems to me that in the event of an alt failure the odds of also having a SECOND unrelated failure... like getting power from batteries to the buss... would be astronomically high.
 
I think by definition, if you have only one battery, you have a single point of failure. It may be a very unlikely one, but it is a single point none the less. There does seem to be varying opinion on whether the alternator(s) will continue to operate properly in the absence of a battery. I've done it a fair bit on old cars with no issues, but I don't really know. However, absence may not be the problem.

If I had a single battery in such a system, it would NOT be a Li-Fe-PO4 battery.

In a previous plane I had a battery failure and the alternator continued just fine. I don't know if it matters, but that setup had an independent voltage regulator - but I have no reason to believe an internal regulator would render a different verdict.
 
+1 to Breister's comments, and they apply to IR alternators, as well.

One battery is *not*, by nature, a single point of failure, though architecture *around* the battery *might* be able to cause a single point of failure.

My definition of single point of failure is a failure that can take down the whole system, with no way to recover.

Charlie
 
I fly IFR for a living and commute to and from work in my RV-10. I am perfectly comfortable with a single alternator, a single AGM battery and a G5 with its backup battery/ADAHRS.

If I lost the alternator, the ships battery should last around 1.5-2 hours. That equates to 200-300 NM range. I cannot imagine flying single engine IFR with widespread weather that is below approach minimums. VFR weather within that range is almost a certainty.

So the worst case is I am down to the G5 with my IPad. That is good for more time than it would take to fly from Florida to Washington DC. and exhaust all my fuel.

Build all the electrical redundancy you want but it will never make a single engine plane an all weather tool.
 
Designing an electrical system to just run off the alternator is quite feasable. The issue is ensuring there is reliable overvoltage protection together with minimizing ripple and high frequency noise on the alternator output. An equally important consideration is ensuring that the output impedence of the alternator is suffiently low to not result in stability issues with the switching regulator and power converters that most electronic instruments have in their power input circuits. Adding large electrolytic capacitors - as Walt showed in his picture should be sufficient if the capacitors are correctly specified to have a low equivelent series resistance. Switching regulators and switching power converters present a negative source impedence to the generator ( they are constant power devices - lower the supply voltage and the input current goes up) so this needs to be considered in the electrical system design.
On a historical note - the Russian MIR ( long since fallen out of orbit) space station reportedly had a few tens of Farads ( not microfarads) of capacitors distributed around the power distribution system to overcome stability problems in equipment power converters due to the source impedence of the small gage distribution wiring they used. Fortunately our cable runs are much shorter.
KT
 
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