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Old 09-04-2022, 07:47 PM
Bicyclops Bicyclops is offline
Join Date: Oct 2012
Location: LA, California
Posts: 410
Default EarthX conversion

I've got 2 electronic ignitions and chose to carry 2 batteries with 1 alternator. My batteries are mounted ahead of and behind the firewall.I've been using the Odyssey PC680s for 5 years and they were not starting the engine as well as they once did and not load testing well. Time to fix that.

I got a couple of EarthX EXT680C batteries in a trade for some labor and decided to take the plunge. Here was my first concern: when they are depleted, they shut off. Not good if you need electrons to keep the engine running. That's why I chose 2 batteries to begin with. So I tested the batteries by pulling 8 amps out of them til they quit using a West Mountain Radio CBAIV. My panel and ignitions draw slightly less than that much once the contactors and exterior lighting on turned off. They each provided almost exactly 95% of their rated capacity before quitting and ran for nearly 90 minutes.

My plan has always been to shut off one battery and keep the other in reserve if/when the alternator fails. This obviously requires active notification of alternator failure which my B&C voltage regulator is supposed to provide via flashing light at 13volts. If I am to hold a battery in reserve, I've got to shut it off as soon as the alternator quits or I'm going to wind up in deep doo doo. Before I swapped out the batteries, I checked that by pulling the field breaker and was surprised that the light didn't flash at 12.8 battery voltage. This would be a real problem with a battery that holds 13.2v at 8A until it's pretty much gone. So I set my EFIS to alarm at 13.8v and also at 3A of charge. On the next test, I got both a visual and audible alarm for both voltage and amps with the field pulled. OK, now we've got a plan.

I did the swap and saved 22.8 pounds. The PC 680 weighs 15.4 and the EXT680 3.5 lb. My CG moved aft about 4 tenths of an inch. This is not only a significant weight savings but I also gained run time post alternator. The lead acid battery would never deliver it's claimed 17 AH at 8 amps by it's very nature. The 5 year old batteries weren't capable of delivering anywhere near an hour each. Another bonus is that the LiFe batteries crank the engine a lot better.

So my current plan (pun intended) is to shut off one battery and also the ignition that is running on it as soon as I'm sure the alternator is really dead. My Lightspeed ignition will compensate by advancing slightly to account for the missing one. Also the ~1amp each draw of the contactors and all the exterior lighting goes. This should give me about 90 minutes runtime on the operational battery. Let's call it an hour to be conservative and then go to the second battery. I've got at least 2 hours and 400 miles before I'm into my last hour of reserves. If I can't find an airport in a big enough town where I can get an alternator overnighted to me in that kind of range..... This is a VFR scenario. If I need pitot heat (and I never have so far in sunny SoCal) then of course I'll need to be landing much sooner.

Ed Holyoke
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Old 09-06-2022, 07:04 AM
Freemasm Freemasm is offline
Join Date: Nov 2017
Location: Orlando
Posts: 956

I'm not following the logic but that 's not uncommon. What exact benefit are you trying to achieve with your methodology? Are you concerned that a bad battery will steal watts from the other? Trying to know when half of your battery capacity has been depleted? So far I see more potential negatives than positives but as mentioned, I'm not following. Let us know your intended objective.
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Old 09-06-2022, 08:56 AM
sahrens's Avatar
sahrens sahrens is offline
Join Date: Jan 2007
Location: Battle Ground WA
Posts: 626
Default Vacuum pad alternator

Would it easier to install an alternator on the vacuum pad? I donít know how much weight savings would result, but it seems you could fly indefinitely on the standby alternator.

Keeping the two battery design with EarthX batteries and the vacuum pad alternator would seem to give you sufficient fault tolerance.
RV-7 N818BG (flying)
Bearhawk Patrol (building)
RV-7 (resurrecting)
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Old 09-06-2022, 09:45 AM
moosepileit moosepileit is offline
Join Date: Dec 2010
Location: Louisville, KY
Posts: 865

I would like to comfortably use all the fuel in my plane, electricity is an artificial limit.
RV-6, bought from builder.
O-320, slider, carb, mags, FP
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