A charging device (10, 100) includes a charging unit (12) and at least one interchangeable, rechargeable energy storage unit (14), in particular an interchangeable battery (18), of a power tool (16). Each energy storage unit (14) has first contact means (24) for contacting the corresponding power tool (16) and additional contact elements (38) for contacting the charging unit (12) in such a way that the charging unit (12) is able to charge the energy storage unit (14) via the additional contact elements (38), both when the unit is connected to the power tool (16) and when it is disconnected from the power tool (16).