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EarthX conversion

Bicyclops

Well Known Member
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
RV-6
 
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.
 
Vacuum pad alternator

Would it easier to install an alternator on the vacuum pad? I don’t 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.
 
Questions answered (long after the fact)

Would it easier to install an alternator on the vacuum pad? I don’t 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.

Sorry for way late reply. Didn't notice that questions had been asked until I looked at it again today. There is currently a spirited discussion on Li batteries, alternators, and EIs going on.

My objective is to be able to safely continue flight to a place of my own choosing after an alternator failure. It would be nice to be able to fly off all my fuel but I'm more concerned with being able to have a predictable amount of time remaining. By switching to the LiFe batteries I increased my predictable time remaining and lowered my empty weight substantially. A win win.

I would only worry about one battery stealing electrons from another if they had different chemistries and the alternator was not running. A Li battery will run itself down if paralleled with a Lead Acid battery, even with no load. If the alternator is running there wouldn't be a problem as the voltage would be higher than either of the batteries' charging threshold. If I knew absolutely that the alternator was done, I could open a contactor to prevent that bleed down. The only problem with that is with the different battery chemistries I'd have different runtime so running one out first wouldn't really tell me much about how much time was left on the other. I actually did a few flights with one of each while testing low voltage and low amps notification before I committed to putting in the second LiFe battery.

To answer why I didn't just put on another alternator, I didn't include the gear drive for the vacuum pad when I built my engine. Dual batteries were always in the cards for my airplane by default. With better options available than the SD8, perhaps I would spring for the gears if I were doing it now. I do believe that dual alternators is a good backup plan for an electrically dependent engine. A single LiFe battery with dual alternators might be slightly iffier especially if it were to be taken offline by an alternator overvoltage event. A friend ran an EarthX out because he failed to turn his main alternator back on after a runup test and his backup alternator was set to come on automatically at a lower voltage than the battery carries under load. His alternator out light never flashed. He only realized he had a problem when his com quit. He had two P-Mags so his engine kept running and he had a backup battery for his EFISs but it didn't power his com or for that matter his flaps and trim.

Another factor which wasn't really part of my consideration at the time but has come up on occasion is that my high compression engine sometimes proved difficult to crank over with a single PC680. I used to start on one with the other covering the panel to prevent brownouts. In fact, there were times as the batteries aged that one just wouldn't do it and I had to use both. I found out that it starts much better on both and brownouts just aren't a problem. The EarthX crank just fine and I use both anyway as I showed up a weak point in my design by using one at a time in that if I left a battery on the E-buss it would blow the 20A fuse when that battery would try and recharge itself through that pathway after engine start before I closed its contactor.

I've been running this setup for several months now and I'm happy with it. Fortunately, I haven't had to test my alternator out strategy for real.

Ed
 
Ed, I have a question out to EarthX now (apparently forwarded to engineering) regarding what sort of current flow to expect if two ETX900s with unequal charge levels are connected. Apparently current can reach very substantial levels. It has a lot of implications for dual battery bus design.
 
Ed, I have a question out to EarthX now (apparently forwarded to engineering) regarding what sort of current flow to expect if two ETX900s with unequal charge levels are connected. Apparently current can reach very substantial levels. It has a lot of implications for dual battery bus design.

Howdy Dan,

Are we talking about one trying to charge the other? I'm interested to hear what you find out. Mine are pretty much always at similar charge levels, I believe. I run the starter on both and they both stay on the main buss during operation. I have observed that after cranking the engine they draw a lot from the alternator - enough to keep the volts down around 13.5 at taxi RPMs. I can't say that I observed the ammeter. I'll look next time. They really want to charge back up right now. By the time I take off, or very shortly after, the voltage is back up over 14 and the alternator can keep up with them. Within a few minutes they appear to be fully charged. There for a short time they are very hungry.

There is a pretty good case to be made for keeping batteries charged up and healthy. I try, but don't always succeed, to fly weekly which seems to me a decent battery maintenance plan. Launching after a jump start for instance would seem to be a really bad idea. Likely to overheat the alternator, possibly to the point of failure, and then where is your battery reserve capacity?

Ed
 
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