EarthX battery inside the firewall?

[Birdseyeview]

I just had my first start of my 912ULS and everything went great so far except the $50 AGM battery I was using as a placeholder doesn't have near enough cranking amperage to turn over the engine very fast (haltingly at best). I'm using this same battery in my quad city Challenger to start a Rotax 503 and it works great for that. The higher compression ratio on 4 cylinders isn't the same scenario and I should have known this was going to happen. However, when I jumped it with my car battery the 912 came to life instantly, showing me that the standard starter I have is very sufficient to start the engine without the need for the insanely expensive HD starter or the "soft start" upgrades. All I need is lots of Cold Cranking Amps and based on the forum here and other local builder testimonials the EarthX batteries seem to be one of the better ways to go (although not the cheapest).

All that aside, my original AGM battery was installed with associated hold-down brackets, solenoid mounting, and wire routings accordingly, all finished and neatly packaged inside the firewall, while being accessible with the top cowl off. At the time I thought keeping the battery cooler on the cabin side of the firewall was a good idea. I see EarthX makes a model that is vented for in cabin installs and it will fit in my existing location nicely as is, but I'm thinking I should probably put it inside the metal containment box that EarthX "Highly Recommends" for in cabin installs. With the smart tech in these newer batteries, and being combined with a smart lithium battery charger, it seems that a battery thermal runaway is unlikely (but not impossible). I don't have access to reliability numbers to do a formal failure modes and effects analysis that might justify the following arrangement but I'm wondering if I put one of EarthX's non-vented battery, without a metal containment box, inside the firewall would an airworthiness inspector object and shoot me down? The difference in cost of the vented battery + containment box vs non-vented batteries without the box is enough to buy a descent lithium smart charger I'll bet. If it's likely that an airworthiness inspector would object to this arrangement I can certainly move the battery to the engine side of the firewall with some rework effort. I know many builders (maybe most builders) with lithium batteries have them located on the engine side of the firewall but I'm looking for experienced opinions both ways before I make any mods and spend a lot of money.

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Also, I see that Kitfox sells the Optimate 9.5A smart charger in their catalog ,so they obviously recommend it, but any strong opinions on other potential good brands of smart chargers to buy would also be appreciated. As always, any constructive ideas/opinions are welcome and appreciated.

[Dave S]

Larry,

Just a thought on the AGM battery - if it was some $50 brand X - possibly the brand, not the technology is responsible for its lack of output.

From the get go, I installed an odyssey 680 AGM for the primary battery which faithfully worked for ten years. I took it off line and replaced it with a new Odyssey due to a slightly lower float voltage and age, not because it wasn't working well enough. The old Odyssey has been used around my shop as a 12 volt power source since that time; and, shows no sign of failing to take a charge or hold it.

While my installation is a little unique with two separate electrical systems, I decided to use a lesser well known brand of AGM battery for the secondary, or backup battery - which pretty much ended up like a case of "those who sell for less apparently know what their crap is worth" - the thing gave up in about a year - significant reduction in ability to hold a charge, then I replaced it with an Odyssey which has worked fine ever since.

Others mileage may vary, but that has been my experience.

[Birdseyeview]

Dave, Thanks for the input on your Odyssey experience. I haven't totally given up on the AMG technology because the PC680 would be a drop in install for me and no need for a new charger. So it's still very attractive from an overall effort and cost standpoint. However, I've been burned by battery / starting issues in the past on my Challenger that I don't want to repeat in my Kitfox and was scared off of the Odyssey a little by some of the negative reviews on Amazon. Most of these are probably due to misapplication but I like the fact that the EarthX has almost twice the CCAs that the Odyssey has. Everything is a trade off and I haven't decided yet whether I'm willing to pay for the EarthX advantages. Oddly enough, the Odyssey weight of 1.5 lbs more than my current AGM doesn't scare me off since my overall weight and CG is in good shape as is.

Does your 912 have the standard starter and/or the soft start feature? Do both your batteries come into play during a normal engine start? How does your 912 start in the winter with the Odyssey (slower but acceptable)?

[Dave S]

Larry,

Admittedly, our S7 has a higher than average empty weight at 886#; however, I have never been concerned about that since I can still fly with full fuel, my wife and 140# of camping gear and food on board. I just try to stay out of the donuts:o.

Regarding your questions, our 912ULS came with the newer version of the starter, but not the soft start. Not too long ago I ended up replacing both modules due to a cut in problem (not that rare on the Ducati modules made during their transition issues back when my engine was made). The new modules came with the soft start feature which is nice; but I regard the soft start feature as "norwegian proofing" (which I can say since my last name ends in ...son) since a person can avoid the kick back issue with proper starting procedure anyway. With the soft start a person can mess up the starting procedure and get away with it.

The airplane does have a cross tie which allows a person to start on either or both batteries; but I have never used it for starting on both batteries. The cross tie feature would be real handy if one of the batteries died out in the boondocks but that has never happened.

FWIW - the airplane always gets preheated if the ambient temp is less than 30 Degrees F so I can't say how it would behave at 10 or 20 degrees, or less, since I have never done that.

The airplane starts just fine down to 30 degrees F. and there is little reduction in cranking speed (hardly noticeable). Since I haven't had any experience with the standard version of the starter, can't say how that would behave under the same circumstances.

If a battery can supply all the power needed, the starter might be the limiting factor on starting RPM. If a jump fixes the starting RPM issue - probably more of a battery issue.

Possibly others can chime in if they switched the earlier version of the starter for the newer version.

For the record, I don't want to steer you away from other informed battery choices, just trying to relate what my experience has been.

[Delta Whisky]

Larry - I too use the PC680 battery. Had one in a RV-6 and in a Cobra (Factory Five). In the RV I put the battery behind the pilot's seat and in the Cobra it was in the trunk. In both cases I routed power via #4 cables. Neither failed to turn over the engine of choice (O-360 or 5L) with gusto even thru Colorado winters.

In my Kitfox all electrical/electronic components (except the ignition units that come on the engine) are behind the firewall for easy access and escape-from-heat reasons. I also have a back up battery from a fairly reputable AGM company to drive the EFIS and EIS during start up and for back up if ever needed. The second battery is way bigger than needed to keep the lights on during startup but since it is there I went a tad larger for backup. The batteries are isolated from each other via a Schottky diode so the backup battery doesn't participate in engine start but the alternator will charge both batteries when running.

My engine has the soft start feature but my experience with long cabled batteries in the RV and Cobra tell me that it need not be part of the consideration.

Pictures taken during the build before wire routing and tie offs were completed:

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[Birdseyeview]

Delta Whiskey, Thanks for your input. I assume you also have the heavy duty starter?

[jrevens]

Larry,

I have a DEKA ETX14 AGM battery made in the USA which came with my kit in 2009. 3 years ago I replaced it just because of age, but I still have it and it holds a charge well. It would probably still be fine in the airplane!
I have a 912ULS with the heavy duty starter, as well as the soft start feature. It turns the engine over very well, summer & winter. That specific battery is used on a lot of motorcycles... CCA is 220, and it is just a 12AH battery. They’ve gone up in price lately, but they are still less than $100. Mine is mounted on the firewall and that’s the perfect spot for me. For whatever reasons my weight & balance situation would be even slightly better if I added additional weight in the nose, so I’m not looking for a lighter battery. My empty weight is 819# and the empty CG is at 9.84”.

I agree fully with Darrel’s comments about a rear mounted battery and relatively long #4 cables, as that’s what I had in my T-18 for 28 years with a 160 hp Lyc. O-320 and a 25AH gell-cell battery.

[Delta Whisky]

Delta Whiskey, Thanks for your input. I assume you also have the heavy duty starter?

Larry - I'm pretty sure it is the heavy duty starter. (Meaning - I think I read it somewhere in the paperwork that came with the engine.) If you know where I can look to be sure, just let me know. Darrel

[Birdseyeview]

After some of the good testimonials about the Odyssey AGM battery it seems that these experiences described so far have all been with more ideal 912 engine start hardware configurations than my current config. Heavy duty starters and/or the soft start feature are all great things and I'm glad you all shared your experiences with me. This discussion has made me more open to go down the Odyssey path and I would probably just get one and try it if someone with the same standard starter and no soft start were to come out with a testimony about their good experience with it. I don't mind spending the extra money to get a stronger battery system if it's actually necessary but the relatively cheap price tag for the AGM is very attractive, if it is sufficient. I'll bet someone out there has my same 912 hardware config (standard starter and no soft start) and has already learned what I'm trying to figure out. Please share your findings.

[jrevens]

Larry - I'm pretty sure it is the heavy duty starter. (Meaning - I think I read it somewhere in the paperwork that came with the engine.) If you know where I can look to be sure, just let me know. Darrel

Darrell - I think the newer "heavy duty" starter has the 2 heavy "ears" sticking out the back. That's one easy way you can tell. I cut one of them off (not shown in picture).

[Delta Whisky]

Thanks John - I then do have the heavy duty starter.

[Birdseyeview]

To close my reason for starting this thread - it turns out the price tag on the Odyssey PC680 battery was attractive enough to buy one and try it. I bought the "Extreme" version at AutoZone for $163 and they said I could bring it back for a refund if it didn't crank my engine over good enough. The "Extreme" version has a metal sleeve around it and comes with a larger terminal bolt adapter (which I didn't use). Although I don't need the metal sleeve since I've mounted the battery on the inside of the firewall, I thought it may come in handy someday if I ever wanted to move the battery forward of the firewall. At 50F ambient temp outside it worked great, even though I don't have the heavy duty starter or the "soft start" feature. The same battery without the metal sleeve and terminal bolt adapter is $126 at my local Autozone. I hope it cranks over this good this coming winter.

[PapuaPilot]

The Odyssey 680 is a good battery. You should be okay as long as it isn't a big distance from the battery to the starter.

[Birdseyeview]

Phil,

Thanks for your comment. I assume you mention the length of the positive cable from the start solenoid to the starter because of a potential voltage drop impact. My battery is mounted just inside of the firewall and so is the start solenoid (see the picture below). I've got 6 gage multi-strand cable, per the build manual schematic from the solenoid to the starter, and all the ground cable from the battery to the engine ground lug. This arrangement isn't as bad a voltage drop as a battery mounted way back in the tail but it's not quite as ideal as a config with the battery and start solenoid mounted on the front of the firewall. However, I'm believing that the difference between my install compared to the ideal should be insignificant due to the fact that my cable length is only a few inches longer (4-5" max) than the ideal. However, I could easily change out the cables from the 6 gage to something larger if the general consensus of the more experienced builders thinks this is the way to go.

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Note how close the battery is to the firewall. The master solenoid is the one most visible in the photo with the start solenoid just below it.

[PapuaPilot]

You have a short run and it shouldn't be a problem. I had an Odyssey battery in the tail originally and there was a pretty big voltage drop. Now I have an EarthX in the same position as you do and it works great.

[jrevens]

If your connections are good it should be just fine there with #6 wire.

[Birdseyeview]

Phil,

Which EarthX model battery did you install?

I'm curious about the impact on your CG when you made the switch from the heavy Odyssey to the light weight EarthX. That must have been a drastic CG impact. How did you compensate for that big a change?

[Delta Whisky]

Recommended Length and Amperage for Battery Cable while maintaining a 2% or less voltage drop at 12 volts


Battery Cable Size
50 Amps
100 Amps
150 Amps
200 Amps
300 Amps


6 Gauge (AWG)
11.8 ft
5.9 ft
4.4 ft
2.9 ft
2.2 ft


Larry - you'll be fine with 6 gauge cable and a PC680. Note the above table is for a 2% or less voltage drop (0.24V). Unless my eyes failed me during research, the PC680 battery is rated at 170A for 30 seconds at 0F and 280A at 32F. (Those values aren't too meaningful because the test routine allows the battery voltage to drop to 7.2 volts but they do give a good indication of available power. Most starter motors work fine until the battery voltage drops to about 10V.) Your starter motor is rated at 0.6kW (about 50A) but the limit is 10 seconds of starter time followed by 2 minutes of cool off time. An extreme PC680 will is spec'd at 520 Amps for 5 seconds. Bottom line, neither the battery nor the cabling should be an issue.

[PapuaPilot]

Phil,

Which EarthX model battery did you install?

I'm curious about the impact on your CG when you made the switch from the heavy Odyssey to the light weight EarthX. That must have been a drastic CG impact. How did you compensate for that big a change?

I got the ETX680C. Yes, it would have made a big change in my CG, but I left the Odyssey battery in the tail. I have been working on getting more stuff in the tail to move the EWCG farther aft. So far I have added autopilot servos and the T3 shock strut, which helped a little. I hope to put a tool kit where the original battery was someday, but I need about 10 lbs. to make the CG work.