You tell smart charger about your car, Battery capacity etc. and it does all the controlling. If you are concerned about keeping the charge to below 80% then you will need to tell it the current state of charge and what you want it charged to but I just connect it and let it do its thing. I've spoken to both the dealer and Lexus UK about keeping the charge state to a given maximum routinely and they say there is no recommended charge state to work to so I'll continue to charge to 100% whenever I want to.
AWC's post in Intelligent charging or not... was marked as the answer