I have an ETX 680 that is about five years old and been in service for 3 1/2 years. It hadn't shown me any real degradation, until it got really cold.
Cold cranking Amps should be 320 at 0F but at perhaps 8F I got about three turns and went into solenoid stuttering. With a few attempts with lots of recovery time in between it did start, but it was very close to failing.
Once it warmed up it behaved very normally. Normal amount of charging for voltage state etc.
So for you Earth-X folks beware the cold cranking capability as it ages.
I just bought my second Earth-X ETX900 and wanted to do a report card on the one I replaced, so this was a timely thread to do it.
I originally purchased the first one 4 and 1/2 years ago (June of 2018) Mine is mounted on the firewall, and I had some issues at first because I had not protected the battery properly from the heat from the convection from the exhaust after shutdown and the hot environment during flight. I knew this, because i wired the fault indicator to one of the discrete inputs to the G3X touch. The BMS told me of the high temp. It got so hot once that the battery case deformed on the bottom. Earth-X (to their credit), re-cased my battery for free and I reinstalled with a full enclosure, and top shroud that bathed the battery with cooling air from the plenum via a 3/4 inch scat hose. No problems since.
Recently When the engine was cold (thick oil), and with my 10 to 1 pistons, the battery would trip on high current sensing, and the BMS would shut off for a few minutes. After a few tries, when it would get over the first hump of compression, it would start normally. The battery was just getting old, I guess.
After consulting with Earth-X, I bought a brand new 900, which has newer software running the BMS, and Wow what a difference a fresh one makes. The voltage will not even drop now enough to kick the G3X screen off line, while cranking.
The new one spin's the 10 to 1 pistons over like crazy, even when cold.
The old battery still would be great for a mower, 4 wheeler, etc. It has over 350 hours flight time, probably 600 plus start cycles on it, and who knows how many turn ons, for loading software in the screens, etc.
This is great product, but they all have a service life, so maybe it's time to upgrade to a newer one.