it sounds like you want to switch from constant current to constant voltage charging. As you are explicitly modelling the power electronics in your system, you need to have a control algorithm that can switch modes. Essentially, one mode should track a current reference (current charging) and one should take a voltage reference (voltage charging). If you don't want to do that level of detail, you need to switch to a behavioral model of the converter.
Please see this page for more guidance: https://www.mathworks.com/help/simscape-battery/ref/batterycccv.html
