If you want to measure how much heat is being transferred from one block to another, you can use this block: Heat Flow Rate Sensor - Ideal heat flow meter - MATLAB.
Alternatively, you can also use this block Probe - Output block variables as signals during simulation - MATLAB to access block variables such as heat flow rate.
If your question is about how to model heat generation inside a Li-ion battery cell (which does not seem to be the case to me), you can refer to Battery Equivalent Circuit block from Simscape Battery, which models both (irreversible) resistive heat and (reversible) entropic heat.Battery Equivalent Circuit - Battery model with electro-thermal dynamics and optional faults - MATLAB.
Best wishes,
Xiangchun
