Sometimes simulation is not enough to secure the proper functionality of peripherals like battery, WLAN and cooling fan.
When it is not feasible to simulate, real components are used for testing with a harness. Electrical testing of different types of peripheral functionalities by automatically connecting components or modules, like battery, WLAN module or cooling fan, to the module being tested.
Contacting test pads with pogo pins or directly with a DUT connector. Peripheral testing can be implemented e.g. by integrating a DUT's functional part into a fixture or by electrically changing required signalling states.