My recent vintage Multiplus has a couple of Aux In that can be used in multiple ways , including to work with a non standard Lithium BMS.
That's precisely what I want to do.
So far I cant find a lot on them. .but measuring I find there is one terminal at 4-5 V DC and one at ground. Can I assume there if I connect them to an open relay contact (or opto isolator) and close that ie connect the 4V to gnd.. then there is a wizard somewhere in the Multiplus that can sense that and do something with it eg shut down the charger?