Thanks for the quick reply! After months of thinking I actually purchased 2x 100ah AGM batteries tonight, I just went to get on the road already! Always room for change in the future.
Too much to concern myself with learning about float charging voltages and what this alternator puts out. I don't want to damage the expensive lithium battery, so I got this pair of trainer batteries.
What you learn with the AGM batteries may, or may not, be applicable to a future lithium setup as each type has their own set of advantages and disadvantages.
Lithiums charge faster, if you can supply enough current to do it, they will go all the way full at nearly full charge rate, and don't need to get full regularly since they are OK at any SOC over time. The can't be charged below freezing or stored below -4*F so heaters may be required. They may be more sensitive to high temps. They prefer not to be charged to totally full, so having a charger that will stop charging early is best. They don't like to be float charged, so a full cutoff/recharge control charger is best.
AGMs need to be taken to full regularly and getting the last 15% SOC takes as much as 6 hours of charge time at absorption. Don't believe the claims that AGMs can be brought to full in 2 hours of driving unless they are only 5-10% down on charge. Most AGM chargers don't do a good job of charging as they are timer or algorithm based and will either over or under charge depending on the system and DOD. Chargers that will do a good job are rare and expensive. Temperature at charging has no low limit, nor does storage if they are fully charged so no heaters required. They like to be float charged so charger can carry the van use while charging, which lithium can't do in most cases. IMO, don't believe the 50% discharge limit on AGM batteries. The actual loss of life, based on energy in and out is more like 15% than the 50% often quoted, even if you go to 20% on every cycle. The discharges average, so a few discharges to 20% SOC loses little life and allows you to have a much smaller battery bank.
We hear all the time about battery chargers that claim to be great for lithium, but don't leave headspace at full or do full cutoff, and about AGM chargers that claim to always charge to full but really don't in the real world. Top line equipment like is used on the very expensive integrated systems is very expensive but would do the best. The big question is risk vs reward on any of the systems. Will better equipment give you enough longer life to justify the cost? That is the decisions that have to be made for either AGM or lithium, or even wet cells which may actually work out the best economically in a lot of cases.
As was mentioned, don't believe the "drop in" sellers for lithium that claim to be able to seamlessly replace lead acid batteries, but also don't believe all the suppliers of "lithium" chargers, regulators, etc that really aren't all that great but just capitalizing on the lithium claim. Look very carefully at how they all work in detail. Also remember all three charging sources, shore, solar, engine alternator need to be suited to the battery type and size, which makes it all even harder to get just right.
