An alternating current power supply assembly which has an extendable operational period and which provides an external power supply for a tool, the power supply assembly comprising: a power inverter unit for converting direct current into alternating current and a removable battery unit which is connectable to the power inverter unit by a coupling (37, 44) to form the power supply assembly and which when so connected forms a discrete unit: characterised in that, the AC power connector is positioned on the power inverter unit; the removable battery unit comprising a battery, a battery management system which is configured to be removeably connected to the power inverter unit via the coupling (37, 44), an integral charger and an AC charging connector wherein; the coupling (37,44) is configured to provide an electrical connection between the power inverter unit and the removeable battery unit and a mechanical coupling which is removeably configured to secure the power inverter unit to the removeable battery unit such that in use, a DC output from the removable battery unit is converted to an AC output from the power supply assembly by the power inverter unit, the operational period of the power supply assembly being extended by replacement of the removeable battery unit when a portion of the energy in the removeable battery unit has been used. a power inverter unit for converting direct current into alternating current and a removable battery unit which is connectable to the power inverter unit by a coupling (37, 44) to form the power supply assembly and which when so connected forms a discrete unit: characterised in that, the AC power connector is positioned on the power inverter unit; the removable battery unit comprising a battery, a battery management system which is configured to be removeably connected to the power inverter unit via the coupling (37, 44), an integral charger and an AC charging connector wherein; the coupling (37,44) is configured to provide an electrical connection between the power inverter unit and the removeable battery unit and a mechanical coupling which is removeably configured to secure the power inverter unit to the removeable battery unit such that in use, a DC output from the removable battery unit is converted to an AC output from the power supply assembly by the power inverter unit, the operational period of the power supply assembly being extended by replacement of the removeable battery unit when a portion of the energy in the removeable battery unit has been used. characterised in that, the AC power connector is positioned on the power inverter unit; the removable battery unit comprising a battery, a battery management system which is configured to be removeably connected to the power inverter unit via the coupling (37, 44), an integral charger and an AC charging connector wherein; the coupling (37,44) is configured to provide an electrical connection between the power inverter unit and the removeable battery unit and a mechanical coupling which is removeably configured to secure the power inverter unit to the removeable battery unit such that in use, a DC output from the removable battery unit is converted to an AC output from the power supply assembly by the power inverter unit, the operational period of the power supply assembly being extended by replacement of the removeable battery unit when a portion of the energy in the removeable battery unit has been used.