I think this depends on what question you would like to answer with your model.
If you just want to study the heat transfer and energy balance across the cooling plate, you can have an input reservoir and an output reservoir, with pumps, cooling plates, & battery in the middle. You'll need a small modification of your current model to do that and it's basically a testing harness of the cooling plate+battery. The conditions of the reservoirs can be linked to your specification for coolant temperature.
If you want to study other factors, like tank size or heat exchanger design, you'll probably need a constant volume chamber to model the storage tank. Here is an example you can take a look at:
https://www.mathworks.com/help/releases/R2021a/physmod/hydro/ug/sscfluids_ev_battery_cooling.html?s_tid=doc_srchtitle
