A new battery question for general discussion-

[Hartstoc]

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

[Carl Froehlich]

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

[hgerhardt]

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

[airguy]

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.

[Hartstoc]

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

[Bicyclops]

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

[Hartstoc]

Originally Posted by hgerhardt

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

[Bevan]

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.

[Steve Melton]

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.

[F1R]

Originally Posted by Steve Melton

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.