Difference between starting and deep cycle service
I don't post much here because as a wannabe builder I don't feel I have much to contribute. However I have been studying lithium iron batteries for deep cycle service in boats, and when you start talking about reserve capacity you begin to get into that realm.
There are thousands of posts on this technology on cruisersforum.com, and anyone interested in these kinds of cells would do well to read those threads. Some of what you read there will run counter to the notion that a drop-in 12v replacement battery can be constructed from LiFe cells.
The reason that marketers get away (legitimately, IMHO) in talking about "lead acid equivalent" capacity is because in deep cycle service you really don't want to discharge a lead acid battery below 50% because it seriously degrades battery life. This is true even with deep cycle cells which have a different plate design than starting cells. Furthermore, you can't rapidly charge past 85% of capacity because you boil off electrolyte. So you realistically wind up with 35% of stated capacity as usable. Worse, if you don't routinely recharge lead acid back to 100%, you suffer sulfation, which degrades capacity. LiFe doesn't suffer from either of these problems--you can discharge them further, and recharge them at higher rates for longer than with lead acid. My next set of (boat) house batteries will likely be LiFe.
Now none of this matters a whit in the aircraft alternator failure scenario. If you need to you're going to use that battery until it's depleted, you don't care how much it's been degraded, and you don't care how long it takes to recharge.
This technology is as different from lead acid as aluminum airframe construction is from composite, and it's critical to separate the hype from the reality.
Doug