Thanks Wrench. I'll bear your comments in mind.

The AGM ( Advanced Glass Matt ) batteries in both aircraft and ground vehicles has been a bit of a disappointment and do not live up to the promise of long life with no maintenance and seem prone to sudden failure with no warning after about 3 years on average. The old wet cell chemistry lasted longer, if maintained.
I believe the problem in aircraft is the wrong type of lead acid battery is fitted i.e. one optimised for starting with high peak current theses type do not like sitting around a better type would be the cross between starter and leisure would give better autonomy if alternator failur. On Lithium batteries never fit a Lithium ion ( LiNMC) these can catch fire only fit LFP ( Lithium Iron Phosphate) these do not catch fire but they are still not direct drop ins as no manufacturer has redesigned any alternator regulators to match the voltage and charge profile of LFPs. If one is fitted the alternator must have over voltage shut down as the battery management system in the battery can under certain conditions disconnect it, this would cause alternator voltage to go wild destroying the electronics and the battery charge current max must be less than the alternator max amp output or it can fry it. A good source of info on lithiums is narrow boat, RV, and Solar PV companies and magazines as these are far ahead of aircraft in the usage of them.

Interesting view. However that is no where near my experience as any battery that gives you only 3 years has either a factory defect, has in-service abuse, or the aircraft electrical system is not to spec regardless of battery chemistry.

And while each battery chemistry has their pros/cons, for your average private aircraft owner, an AGM battery provides the best bang for buck in general IMO with my recip customers averaging 7+ years life and turbine customers 5+ years.

But keep in mind, even a lithium battery will not last with abuse or a poor performing electrical system.