What about in the case of charging? If one recalls the last time when charging the cellphone, an adapter to the cellphone must be plugged in. While the adapter is connected to the phone, it is still on until it is fully charged. Shown here is how that works and why it is important to have low Rds(on). First, connect the AC to DC adapter to the device, it will power on the device. If control IC gives an ok to the charge FET, the battery will also be charged. Then a few hours later, the phone battery is fully charged. Now same as the discharging, there are two main power losses from the body diode of discharge FET and Rds(on) of the charge FET. Body diode conduction loss is relatively smaller and will be ignored here. 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 battery voltage) will be close to Vin (adapter output voltage).