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A new battery question for general discussion-

Hartstoc

Well Known Member
I’m in the process of upgrading my 7A to dual Lightspeed EI, and pondering ways to add appropriate reliability and true redundancy to my electrical system. On that front, I’ve pretty well concluded that a dual B&C alternator, dual-regulator, single battery system makes the most sense(not the subject of this thread-has been well covered in many others, but I will note that my plan is not to have a primary and an automated backup alternator, but instead a dual primaries that can be manually switched back and forth with one always completely inactive.)

I’m also removing the carb in favor of an Airmotive Performance Fuel injection system, and it has occured to me that a truely redundant electrical system could also support dual electric fuel pumps and enable me to eliminate the engine driven pump.

My question has to do with the choice of battery, which I will be planning to replace annually. Like MANY RV’s, mine is now equipped with an Odessey PC680, which is admitedly a solid, economical, and generally adequate unit weighing about 15pounds, but with fuel injection a bit more Ampacity could ocassionally come in handy. Is there something special about the PC680 that makes it uniquely well suited to aircraft?

Looking at the entire list of Odessey “extreme” batteries, there are several that appear to have considerably better specs in terms of capacity, reserve, and cranking power versus weight. In particular, the PC950 weighs just a few more pounds but has far greater cranking reserves than the 680. Link below- any experiences or comments on this?

https://shop.odysseybattery.com/p/pc950

Also, any battery would obviously be happier aft of the firewall instead of the usual forward installations. Would it be inviting disaster to mount these recombinant, near-zero off-gassing batteries on the people side of the wall? Thanks in advance for any shared wisdom!-Otis
 
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Two specific responses:
- I find the PC-625 a better form factor than the PC-680, it is also slightly lighter and has a little more capacity than the PC-680.
- No concern at all on mounting the batteries aft of the firewall. The RV-8A had one on each side of the floor just aft of the firewall, the RV-10 had both on the normal location aft of the baggage compartment, and the new RV-8 project has one battery on the right side aft of the firewall and one in the normal location aft of the baggage compartment. I?m not a fan of mounting any battery on the engine side of the firewall.

Three side comments:
- If you are running dual Lightspeed ignitions you must have some kind of backup battery setup (I refer you to the Lightspeed installalation instructions).
- While I recognize the trend is for dual electric fuel pumps, I find no benefits that outweigh the downside of making your engine even more depending on electrical power.
- Your dual alternator idea will provide power if you loose one of the alternators - but that is the only electrical fault it protects against. All other failure modes leave you with a stopped engine. As discribed, I offer your statement that you have a ?truely redraft electrical system? is not accurate.

Carl
 
I have an RV-6 with dual Lightspeed and FI. I wired mine similar to Bob Nuckolls' Z-35 schematic (single alternator with 2nd small (non-cranking) battery). RV-6's normally have the battery on the cold side of the firewall, so locating it there is not a problem.

I'd caution you to rethink your single-battery scheme only because I've had an AGM battery (WestCo) with a cell connector failure. One second it was cranking beautifully, the next it was totally dead with zero volts. Alternators need a battery in the circuit to reliably 'alternate', as changing electrical load can momentarily drop bus voltage to where the alternator field collapses. PM alternators may not have that issue; not sure about those.

Heinrich
 
And for every "Your gonna die" thread about electrical failure there are a large number of installations that are working fine.

This is experimental aviation. You are clear to deviate from the norm - BUT - recognize that our sport is very unforgiving of failure. I'm pretty far out on the end of the tree limb myself with significant deviations from the norm with my electrical and fuel systems.

Standard aircraft electrical and fuel system designs exist in their current format due to several decades of lessons learned in blood. Changes are possible, but should be looked at VERY carefully and in-depth failure chain analysis and risk evaluation is needed.

I'm an advocate for experimentation, and against fear-mongering - but I'll also caution against making changes unless you are fully educated on the Plan A, Plan B, and Plan C of those changes.

Go forth - but go forth carefully.
 
Feedback much appreciated!

Thanks everyone for the responses, all duly noted and all to be carefully considered. Especially good to hear from someone who has put an AGM on the pax side of the firewall. I’ve been pondering this issue of redundancy in an all-electric installation for quite some time, and it seems to be a “work in progress” for most of us. I’ve personally been oscillating between “one alternator, two batteries” and “one battery, two alternators”. The latter seems to be winning- with a belt-driven 40-amp B&C as #1 primary and a 20-40Amp B&C gear driven as primary #2. I’m all-LED and lean-IFR on the electronic loads.

I’ve thought a lot about how to set up a backup battery and keep hitting the walls: “How can you be sure just how much juice is really waiting in that thing?” And “Will it really get you out of harm’s way if you are in the smoke or the middle of nowhere when the charging system goes belly up? Also, really meaningfull backup batteries are very heavy. I’m dumping two magnetos and an engine driven pump, and I think I can reach my goal with the net addition of just 5-10 pounds, depending on the final choice of battery.

I actually think the system I invision for my bird will be quadruply redundant. Actual failures of a high-quality, reasonably new but proven batteries are quite rare, so I plan to have a good one living in a happy environment as as significant store of energy in case it takes some time for me to notice the flashing lights/EFIS signalling a low-voltage condition, the prominant voltmeter indicating <13V, or that an overvoltage condition has crowbarred my #1 alternator. When I do finally wake up to the fact that I’ve got a “situation” to deal with, I will able to kill the main bus, bring the fully redundant #2 charging system on-line, and then start exploring the main bus for problems. All the while, both EI’s have been suckling juice from the battery, to which they are both directly switched. With the fresh #2 charging system awakened, and any offending circuits disabled, I should be good to go for the duration of fuel on board regardlessof what else is broken. ALT switch will be three position: left #1, center off, right #2.

Please regard that last paragraph as a fair-game target and open fire. Any direct hits will be regarded as huge favors! -Otis
 
Single point of failure

Consider not having both alternators on one switch. You don't even really need switches. Use pullable field breakers and only push in the one you need at any given time.

Ed Holyoke
 
I have an RV-6 with dual Lightspeed and FI. I wired mine similar to Bob Nuckolls' Z-35 schematic (single alternator with 2nd small (non-cranking) battery). RV-6's normally have the battery on the cold side of the firewall, so locating it there is not a problem.

I'd caution you to rethink your single-battery scheme only because I've had an AGM battery (WestCo) with a cell connector failure. One second it was cranking beautifully, the next it was totally dead with zero volts. Alternators need a battery in the circuit to reliably 'alternate', as changing electrical load can momentarily drop bus voltage to where the alternator field collapses. PM alternators may not have that issue; not sure about those.

Heinrich

Heinrich?s note, and Carl?s as well, are both points well taken. I certainly see the advantage of having a backup battery for dual EI?s, but the notion of eliminating the engine driven fuel pump pretty much implies a backup alternator because of their 5-amp draw. I guess the ultimate solution and answer to Heinrich?s concern would be two smaller batteries in parallel, a complication I was hoping to avoid but will reconsider. If I decide to keep the engine driven pump, I agree the Bob Nuckolls approach makes a lot of sense.
- Otis
 
I’m in the “main alt, small backup alt, single battery” camp. The thing with batteries is that you never know how much is in the reservoir at any one time. 50% full? 80%? This is unacceptable to me. On the other hand, a backup alternator can supply unlimited power at a given rate. This alternator should be of high quality, held in reserve and sized to provide the needs of your essential loads to complete the flight.

Technically, I have 4 alternators onboard. Overkill? Maybe but I’m not going down because my only alternator and unknown reserve (battery) fail.

Also, have a close look at where your single point failures are and their consequences. No one failure should cause you to break a sweat.

Bevan.
 
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Nov 26, Southwest of Cincy, a guy had a EarthX battery and Lightspeed EI. the EarthX failed and he became a glider but was able to get it restarted using a backup battery.
 
Nov 26, Southwest of Cincy, a guy had a EarthX battery and Lightspeed EI. the EarthX failed and he became a glider but was able to get it restarted using a backup battery.

Good thing that was not right after take off. Any report if was an alternator over voltage or exactly what was the root of the cause? I doubt that the battery failed, but more probable is that it simply shut down to protect itself.
 
Heinrich?s note, and Carl?s as well, are both points well taken. I certainly see the advantage of having a backup battery for dual EI?s, but the notion of eliminating the engine driven fuel pump pretty much implies a backup alternator because of their 5-amp draw. I guess the ultimate solution and answer to Heinrich?s concern would be two smaller batteries in parallel, a complication I was hoping to avoid but will reconsider. If I decide to keep the engine driven pump, I agree the Bob Nuckolls approach makes a lot of sense.
- Otis

I offer two identical batteries in parallel on a single alternator, if thoughtfully designed, is the simplest, lightest and most robust path for you to take. Consider your two master solenoids as a path for engine start and battery charge - and nothing else. Now expand the battery buss concept so that each battery has a direct feed to one of your ignitions, and another feed via a 30 amp relay to half of your panel.

Now when something bad happens the only pilot action is to open both master solenoids to establish the most stable condition. At that point you decide what you want to do - while you are enjoying the flight to a convenient place to land. Two PC-625 batteries with no alternator will provide 1.5 to 2 hours of full IFR flight - more depending on you panel draw.

As you correctly stated, modern batteries are the most reliable thing in your airplane ASSUMING you have maintained them and have not abused them (like jump starting a dead battery). This is not what you should be worried about. All the connections and solenoids that you added provide the single point failures that will leave you with a dark panel and a stopped fan.

I offer a simple exercise - look at every connection, switch and breaker and identifying what will happen it that element fails (include ground connections). If you cannot accept the result, you need to reconsider your design. Everything fails, as such your design needs to ?gracefully degrade? with no pilot action, not totally fail.

Carl
Stepping off my soap box.....
 
Good thing that was not right after take off. Any report if was an alternator over voltage or exactly what was the root of the cause? I doubt that the battery failed, but more probable is that it simply shut down to protect itself.

Distinction without a difference. If battery energy isn't available due to the battery itself, then there's your failure.

I love the idea of more energy-dense batteries, but we should be aware or all ramifications when we make a change that isn't a simple 'drop in' replacement for traditional components and systems.
 
Here is my take on this. I'm running dual lightspeeds. Single alt and single battery. A few points. I am installing a small backup dedicated to the ei units, but have no fears of flying it until I get that done.

First, what is the aircraft mission? VFR, IFR, hard IFR. VFR, how far is a suitable airport? IFR could be a lot farther. Hard IFR, wrong aircraft in my book.

First consideration what battery. I'm in the old school sealed la battery. They are simple, reliable, and for the most part, the failure modes are known. The odds of having a working battery go to zero volts are pretty slim.From my discussions with klauss the unit should run down to about 6 volts or so. The li batteries have strict operation rules and I think more total failure modes. I would never run a li without a backup.

The best way to set up dual electronics is kiss. Battery, fuse, switch, unit. Separate components for each system.

For the back up, a small 5 or 6 amp/hr la with diode isolation. A guarded switch to select source. That's it. Use a selector switch for each unit and the odds of total power failure to the units is slim.

For a VFR or light IFR aircraft I am confident in this setup.

Bob burns
Rv-4 n82rb
 
Bob Nuckells is back!-

I offer two identical batteries in parallel on a single alternator, if thoughtfully designed, is the simplest, lightest and most robust path for you to take. Consider your two master solenoids as a path for engine start and battery charge - and nothing else. Now expand the battery buss concept so that each battery has a direct feed to one of your ignitions, and another feed via a 30 amp relay to half of your panel.

Now when something bad happens the only pilot action is to open both master solenoids to establish the most stable condition. At that point you decide what you want to do - while you are enjoying the flight to a convenient place to land. Two PC-625 batteries with no alternator will provide 1.5 to 2 hours of full IFR flight - more depending on you panel draw.

As you correctly stated, modern batteries are the most reliable thing in your airplane ASSUMING you have maintained them and have not abused them (like jump starting a dead battery). This is not what you should be worried about. All the connections and solenoids that you added provide the single point failures that will leave you with a dark panel and a stopped fan.

I offer a simple exercise - look at every connection, switch and breaker and identifying what will happen it that element fails (include ground connections). If you cannot accept the result, you need to reconsider your design. Everything fails, as such your design needs to ?gracefully degrade? with no pilot action, not totally fail.

Carl
Stepping off my soap box.....

Carl- I say ?Bingo? to your ideas here, so thank you! This is actually the approach that I?ve given the most thought to, but I do feel that one alternator, however good, implies an engine driven fuel pump, as an electric pump draws 5-7 amps, and batteries alone can?t be counted on for what is going to amount to a near 10 amp essential survival load of one pump + one EI. I?m reluctantly letting go of the idea of removing the engine driven pump.

That does really open the door to a somewhat modified Bob Nuckolls solution using, as you suggest, two identical batteries. Each should be just capable of starting the engine and each with its own dedicated contactor to the main bus. I don?t think there is a need to divide the panel between the batteries, both or either can connect directly to it. One penalty will be the need for a second starter solenoid to allow both batteries to routinely participate in engine starts but both can be lower capacity units. The master contactors can both connect to the full bus. I think the best design makes the two batteries completely interchangeable, and allows all systems to operate with either or both masters on.

The lifesaver mode of this design, as you say, is both masters off, leaving each battery with the very simple task of supporting a single ignition module, as these should be switched directly to the batteries. Your engine still runs, and you are now in a position to assess reacources and decide how much juice you can afford while getting to your destination. (I could see my iPad pro running Foreflight with synthetic vision coming in handy in this situation, so I always carry that with its own power brick.)

This simplest form does leave one single failure mode worth noting- a pilot astute enough to notice that there is a problem with the electrical system before both batteries are dead. I?m willing to take that responsibility with a well-designed warning system in place, but there are strategies that can charge either battery(at sub-par voltage)through diodes even with its master open that might be considered. That would suggest a procedure of turning off one master contactor some time after engine start. It would also satisfy Klaus? installation criteria for the lightspeeds.

BTW- One main reason for my choice the Lightspeed PlasmaIII?s is their anti-runaway option, which shuts them down above about 3KRPM. I?m also installing a Hartzell Comoposite CS prop on my 7A, and I?ve heard too many expensive oversped prop stories. I also like that they aparently have a more conservative advance at high MP than the P-mags.- Otis
 
This is a very old discussion for the alt engine guys, since most have been running electrically dependent engines for much longer than Lyc guys.

Most of the VFR guys I'm familiar with choose either 2 batteries (somewhat heavy, limited duration) or 2 alternators (lighter, unlimited endurance, but slightly more complicated to wire up).

My thought is that if IFR is in the mix, there's no substitute for 2 alts. And with 2 alts and one 'traditional' AGM battery, you've got about the same risk of complete electron loss as airframe structural failure.

I have the luxury of running 2 identical alternators (alt engine), so I'm set up with 2 alts & one battery. Unlimited endurance, lighter than an additional battery, no complicated load shedding for endurance, and with the graceful failure mode of AGMs, I won't worry any more about battery failure than wing spar failure. I just have an extra control for alt 2, and a bus tie switch between engine bus and airframe bus, so the engine bus & airframe bus both an alternate power path.

I no longer bother with posting pics here, but I'll be glad to email a rough drawing if anyone's interested.

FWIW...
 
Respect your points, but-

This is a very old discussion for the alt engine guys, since most have been running electrically dependent engines for much longer than Lyc guys.

Most of the VFR guys I'm familiar with choose either 2 batteries (somewhat heavy, limited duration) or 2 alternators (lighter, unlimited endurance, but slightly more complicated to wire up).

My thought is that if IFR is in the mix, there's no substitute for 2 alts. And with 2 alts and one 'traditional' AGM battery, you've got about the same risk of complete electron loss as airframe structural failure.

I have the luxury of running 2 identical alternators (alt engine), so I'm set up with 2 alts & one battery. Unlimited endurance, lighter than an additional battery, no complicated load shedding for endurance, and with the graceful failure mode of AGMs, I won't worry any more about battery failure than wing spar failure. I just have an extra control for alt 2, and a bus tie switch between engine bus and airframe bus, so the engine bus & airframe bus both an alternate power path.

I no longer bother with posting pics here, but I'll be glad to email a rough drawing if anyone's interested.

FWIW...

Charlie- I do respect the setup you describe, but would say that if you also depend on an electric fuel pump and/or electronic injectors, some kind of alternate battery, even a tiny one; that you could put on the main bus in the unlikely event of a main battery failure could save the day. As someone pointed out, the alternator really does need to work into the ballast provided by a battery and some voltage is needed to activate the field.

I?m not sure the IFR/VFR itself makes much difference. Even flying In CAVU conditions, if you are remote and in the mountains you need an operational powerplant, ideally one offering more options than just landing the nearest strip, which itself can become a real PIA.

I?m trying to satisfy a number of goals here- elimination of single-point electrical failures that could stop the engine, ample battery backup that can also greatly enhance routine cranking power- identical batteries one of which gets replaced annually and can individually run the whole show, minimal overall weight such that all these goals are met, etc.

Most, but not all of this thread rehashes things in older ones, but judging by the response, these things do still occupy our thoughts, and there are always ways to incrementally optimize and improve system redundancy. I?m certainly grateful for all the wisdom that has been shared here- VAF at its best!-Otis
 
Points of Failure

I have to echo what Carl and Bevan wrote - be careful about your ground wires, and check every single connection and ask yourself - if this fails, how do I deal with it? A failure could be a bolt comes loose, or a wire breaks, or something shorts against it, or whatever.

On the ground wires, be sure to have 2 nice fat ground wires from the engine back to the airframe/battery.

GROUND-BRAID-w-LUG-L-DSC02195.jpg


http://www.rv8.ch/ground-point-on-firewall/
 
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I?m reluctantly letting go of the idea of removing the engine driven pump.

You're right - keep the engine driven pump.

One penalty will be the need for a second starter solenoid to allow both batteries to routinely participate in engine starts but both can be lower capacity units. The master contactors can both connect to the full bus. I think the best design makes the two batteries completely interchangeable, and allows all systems to operate with either or both masters on.

I've got 2 PC680s, each with its own contactor. You don't need a second starter contactor. Hook the switched side of each battery contactor to the starter contactor and you can use either or both for starting and charge either or both from the alternator(s). I have each battery driving an ignition directly and each capable of powering the E-buss when its contactor is open (2pole battery switches - OFF-Ebuss-ON).

This simplest form does leave one single failure mode worth noting- a pilot astute enough to notice that there is a problem with the electrical system before both batteries are dead. I?m willing to take that responsibility with a well-designed warning system in place, but there are strategies that can charge either battery(at sub-par voltage)through diodes even with its master open that might be considered. That would suggest a procedure of turning off one master contactor some time after engine start. It would also satisfy Klaus? installation criteria for the lightspeeds.

You probably don't want to normally run with one battery contactor open. Keep the battery on the buss so it is charging. If you use the BandC regulator(s), you will have active notification of alternator failure, overvoltage protection, and plenty of time to fire up the other alternator or transition to battery only ops.

Ed Holyoke
 
Help me understand failure modes ?

When a battery fails, does it become a large current draw creating heat or does it become a high resistance / Open circuit ?
As discussed some previously, can a well regulated / OV protected alternator operate ignition without a battery to buffer the discharge ?
 
Failure modes: All of the above are possible. But with an AGM that hasn't been abused, the failure mode is typically a simple loss of reserve capacity. And in this case, 'typical' means not much worse odds than a wing failure. Everybody needs to define what will make them comfortable with redundancy. We've all trusted a single carb (even though they have a measurable failure rate), and a single set of wings.

I've used AGM batteries in planes for over 2 decades, & paid attention to accounts of failures, and I can't recall any failures I've heard about where the battery itself either shorted out or went full open circuit on its own, except the one incident mentioned earlier on this forum (unverified).

My personal trust level is to assign the same level of risk to inflight battery failure that I assign to the airframe. Mechanisms like alternators understandably have a much higher probability of failure; that's what most people are looking at when they try to build in redundancy. Most (not all, obviously) consider the battery to be the backup for the charging system**, and a 2nd battery as an extension of the backup; not a 'plan C'.

**edit: That's really the wrong term for the alternator in this specific instance. It's not really doing charging duties that we're worried about, since the battery gets fully recharged within a few minutes of a normal start sequence (certainly before the end of a typical cross country climb profile). In normal flight, the alternator is the source of all electrons. Perhaps 'electrical generating system' would be a better term; though not very elegant.

As to alternator stability without a battery, there's a lot of interweb lore, but little documented evidence to support it (either way). I think that Dan H tried it and found a small instability in voltage, but it was on the order of a volt or so (well within reason for safe operation). Having said that, I would suspect that each *system* could react differently, depending on the alternator, the specific regulator in use, length/position of wiring, etc. The 'dynamo' style alternators (permanent magnet) will all work without a battery. Regular wound field alternators theoretically have no reason to stop working when the battery is removed, *unless* the field is interrupted and/or it's disconnected from the a/c system. Consider that a fully charged battery is effectively an open circuit to the alternator, and all the electrons from the alternator are actually flowing into the system, with a few making their way back through the regulator to power the alternator's field.

My opinion; worth what you paid....
 
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RV7Charlie question

Charlie, let?s drill down into a battery deficiency. You got the engine started and the alternator is attempting to recharge the battery. But, your old AGM has been losing capacity unnoticed. There has been discussion about relays to disconnect the battery from main buss,EBuss/Aux buss etc. my question is what happens to Alt output for charging battery #2 and maintaining 6+ volts for ignition. Must you disconnect the primary battery to avoid problems and thus potentially eliminate one ignition system?
 
1st, the disclaimer: I'm no authority.

It sounds like your concern is the battery drawing higher than normal current as it loses capacity. I don't think that will be the case. My understanding is that the loss of capacity means that it just isn't accepting and storing as much energy; not that it's...well... shorting out. Think about any your other 'stuff' that has a starting battery. The 1st indication of a bad battery is when it fails to start the [whatever]. Nothing in the car/boat/tractor has been affected. But if you'd done a capacity check a month before that battery failed to crank that engine, you'd have seen greatly reduced capacity to run a continuous load, like the radio, engine controller, etc. In a car, for instance, after an alternator failure a fresh battery might let you drive the car home from, say, a hundred miles away, but the old tired one might only get you 10 miles before the engine dies. There was no effect on the ability of the alternator to supply any load demands as the battery's capacity declined. (BTW, it's extremely unlikely that the battery killed the alternator in that example.)

Now, all the above leads to the statement about the old AGM in the plane losing capacity unnoticed. That really shouldn't happen in an a/c, especially if it's an electrically dependent a/c. The Aeroelectric book discusses annual capacity testing (required for type-cert a/c), and also discusses things like, (in a 2 battery system) replacing one battery every year or two and moving the removed battery to ground duty in some other machine. Point being, we shouldn't have old AGMs with undetected reduced capacity flying in our electrically dependent planes.

I keep coming back to asking if these are solutions in search of problems. Consider how many IFR a/c there are out there that have flown for many decades using a single battery, even in the days of failure prone generators. Even with old flooded cell batteries, the in flight battery failure rate was very low, and I'd submit that with AGM, it's even lower. It seems to me that this concern about protecting the system from battery failures only arose after we started seeing meltdowns and internal-BMS-driven disconnects in lithium-based batteries. If one is running a battery that has a record of meltdown or self-disconnect in an electrically dependent a/c, then contingencies are certainly warranted.

Again, no Holiday Inns in my history, and worth what you paid...
 
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