Anyone done this ?

[Navi2016]

https://www.zwerfcat.nl/en/lithium-hybrid.html

[smac999]

sort of but with a 1-2-all switch where the batteries meet. since the bms never opens unless there is a problem.

"When the lithium battery is fully charged it is taken offline by the BMS" this is incorrect. the BMS will not open unless there is an problem.

at dock agm battery only. agm floats and lithium can sit. at anchor lithium battery only, no need to drain the agm. while motoring on both the agm protects the alt from a bms open.

[GordMay]

For those who don't like to follow 'blind' links, it's about:
Lithium/Lead-acid hybrid battery systems

[flee27]

Along the same theory. This guy does a good job of explaining the use a lead and lithium. He also has a BMS to help accomplish a system with the 2 battery types. Adds some more info of the subject. Like most things boating it is always a 2 sided sword. All about weighting the pros and cons to make the best choice for your boat.

https://www.emilyandclarksadventure.com/bbms

Foster