if you are generating code for a controller, you should use a model reference. This will allow you to have a simulation where the plant model (power gui model of the battery pack) runs at a smaller time step, in this case 50e-6, while allowing you to generate code for a controller that runs at a slower time step. I would recommend starting here for simulink basics:
Here is a starting link for model references:
If you are targeting a real-time system for the controller, you will need Simulink Coder. If you are targetting an embedded controller, you will need Embedded Coder. On the off chance you are using an FPGA, you will use HLD coder instead.
Your sample time for the controller should be the same rate you want to run your control algorithm at. For a typical battery SOC, this should be slower than 50 e-6, while you should leave your plant model at 50e-6. You can use the rate transition block to control what parts of your model run at different time steps.