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.

cabin side AGM battery.jpg

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.