There seems to be some confusion ,
in a boat or maybe an RV , tail current is primarily the load current experienced during charging , it’s not a fixed value , that’s the issue. Without any load , tail currents in LA are often very small , 1A or less.
Since a charge source can’t sense total battery current , yiu have a problem of how to determine when to leave absorption mode. This problem still to this day dogs “smart “ chargers. ( and I never understood why a battery shunt option wasnt offered integrated into the charger ) and has resulted in some chargers “ boiling “ battery banks by staying in absorption mode too long.
So without a shunt , all you can do is set a “ nominal “ value based on the bank size and your knowledge of the load currents that might exist during the absorption period. Good smart chargers now use several factors to exit absorption namely :
A termination voltage setting
A maximum allowed absorption timer
A fixed tail current threshold , the charge current must be lower then this for a minimum time.
The problem is , if you have a dynamic tail current , a classic being a fridge cycling on or off , it’s very hard to pick a fixed threshold , you find either absorption mode is exited too soon ( tail current setting too high ), resulting in undercharged batteries ( really bad for AGMs) or the dynamic load is such that absorption mode never exits until the protective timer trips ( tail current setting too low ) resulting in too long an absorption mode , potentially damaging batteries
Now along came victron with a family of charging devices , these can be network together via the Cerbo /Venus central control system , or within a certain sub set of products , Bluetooth networking , called VE.Smart can be used without needing a central controller.
Now if you add a compatible battery shunt , Smartshunt or BMV712 , you can now feed back battery voltage ( not really terribly useful ) battery total current ( very useful ) and optionally battery temp ( quite useful ) to any devices participating in the VE.smart network. ( or via Cerbo/Venus )
Now the charge source can eliminate load currents from its charge currents , say during absorption mode the charger sees 10 A , and the battery shunts reports -6A ,that means 4A of the chargers output is flowing into the load . The beauty is the calculation is dynamic , as the load varies the charger can now determine true battery tail current. This computed value can then be compared against the tail current setting in the charger. This means that absorption mode exit is based on net true battery current , rather as if the charger was solely charging an unloaded battery. This means absorption mode exit is not affected by load currents.
What’s often over looked in VE.Smart , is then if you have raised the tail current settings because the charger was in standalone mode , and then introduce VE.Smart you need to reduce the tail current setting to a lower ( usually default ) setting otherwise the charger will exit absorption mode way too soon.
Another advantage with VE,smart , is that the charge sources ,Smartsolar mppt and Blue charge IP22 can synchronise their charge modes. One charger is elected as a master and as it changes from bulk to absorption to float , it commands the “ slaves” to switch modes , this results in consistent charge mode transition and termination