Shown here is a simplified battery circuitry. Initially, if the battery control IC does not tell the discharge FET to turn on, there will be no current flowing through the battery-powered device, therefore, the device is off. Now, if the control IC turns on the discharge FET, current will travel from the battery to the load, passing through the body diode of the charge FET and through the discharge FET then back to the battery. Again, life is not ideal, there are two main power losses; one from the body diode and one from the Rds(on) of the discharge FET. The body diode conduction loss is normally much lower, so it will be ignored for now. If a MOSFET that has lower Rds(on) is chosen, the power loss can be minimized. As mentioned earlier, the lower the Rds(on), the lower the voltage drop, which means Vout (the device voltage) will be close to Vin (battery voltage).