A battery charger (20, 20A) comprising: a housing including one or more charging ports (25, 25A), each of the one or more charging ports (25, 25A) configured to receive a battery pack; a power supply module configured to be electrically connected to a power source and configured to supply power to the battery charger (20, 20A); a controller configured to receive an indication of a presence of the battery pack in one of the one or more charging ports (25, 25A); at least one protection circuit including at least one semiconductor switch; and a battery pack detection device (410, 500) positioned within at least one of the one or more charging ports (25, 25A) and electrically connected to the controller, the battery pack detection device (410, 500) including: a first conductive part (415, 535); a second conductive part (420, 545); a first terminal (430, 530) arranged to couple to the first conductive part (415, 535); and a second terminal (480, 540) arranged to couple to the second conductive part (420, 545); characterised in that: the at least one semiconductor switch configured to prevent a charging current from being supplied to a charging port (25, 25A) when a defect condition related to the charging port (25, 25A) is detected; wherein a voltage of the first terminal (430, 530) is a battery charger reference voltage, a voltage of the second terminal (480, 540) is a battery charger control voltage, and the first conductive part (415, 535) and the second conductive part (420, 545) are brought into electrical connection with one another when the battery pack is inserted into the at least one of the one or more charging ports (25, 25A), and wherein the voltage of the second terminal (480, 540) is reduced from the battery charger control voltage to the battery charger reference voltage after being brought into electrical connection with the first terminal (430, 530). a housing including one or more charging ports (25, 25A), each of the one or more charging ports (25, 25A) configured to receive a battery pack; a power supply module configured to be electrically connected to a power source and configured to supply power to the battery charger (20, 20A); a controller configured to receive an indication of a presence of the battery pack in one of the one or more charging ports (25, 25A); at least one protection circuit including at least one semiconductor switch; and a battery pack detection device (410, 500) positioned within at least one of the one or more charging ports (25, 25A) and electrically connected to the controller, the battery pack detection device (410, 500) including: a first conductive part (415, 535); a second conductive part (420, 545); a first terminal (430, 530) arranged to couple to the first conductive part (415, 535); and a second terminal (480, 540) arranged to couple to the second conductive part (420, 545); a first conductive part (415, 535); a second conductive part (420, 545); a first terminal (430, 530) arranged to couple to the first conductive part (415, 535); and a second terminal (480, 540) arranged to couple to the second conductive part (420, 545); characterised in that: the at least one semiconductor switch configured to prevent a charging current from being supplied to a charging port (25, 25A) when a defect condition related to the charging port (25, 25A) is detected; wherein a voltage of the first terminal (430, 530) is a battery charger reference voltage, a voltage of the second terminal (480, 540) is a battery charger control voltage, and the first conductive part (415, 535) and the second conductive part (420, 545) are brought into electrical connection with one another when the battery pack is inserted into the at least one of the one or more charging ports (25, 25A), and wherein the voltage of the second terminal (480, 540) is reduced from the battery charger control voltage to the battery charger reference voltage after being brought into electrical connection with the first terminal (430, 530). the at least one semiconductor switch configured to prevent a charging current from being supplied to a charging port (25, 25A) when a defect condition related to the charging port (25, 25A) is detected; wherein a voltage of the first terminal (430, 530) is a battery charger reference voltage, a voltage of the second terminal (480, 540) is a battery charger control voltage, and the first conductive part (415, 535) and the second conductive part (420, 545) are brought into electrical connection with one another when the battery pack is inserted into the at least one of the one or more charging ports (25, 25A), and wherein the voltage of the second terminal (480, 540) is reduced from the battery charger control voltage to the battery charger reference voltage after being brought into electrical connection with the first terminal (430, 530).