A voltage negative to ground connected on the input (charger with reverse polarity) will pull the source of the discharge FET to negative voltage and with the ground potential on the gate the FET will likely be turned on. Since the diode of the Charge FET will conduct a discharge current, a large current will in this case flow out of the battery cell(s) with the ATmega16HVA unable to stop it. A FET (Q2) on the pack input which pulls the source of the discharge FET to the negative input voltage is included in the design to avoid this situation. This is an optional part in a design but included in the main area.