Simulator - Body Battery?

For things you get from SensorHistory (like Body Battery, Temperature, pressure, HR) the sim uses canned data.  It's not just a single value, but a history of values.  The best way to test the responsiveness is on a real device, where in the case of a watch face you also have low power/high power to see and in all cases, how often the history is updated it's probably only every updates every few minutes.  You can compare two "when" values and see the difference to see how often a sample is saved.