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A power supply unit for supplying power to a device has a rechargeable, main battery; a charging arrangement for charging the main battery; a non-rechargeable back-up battery; load terminals for connection to a load; and a control unit for controlling supply of power to the load primarily from the m

A power supply unit for supplying power to a device has a rechargeable, main battery; a charging arrangement for charging the main battery; a non-rechargeable back-up battery; load terminals for connection to a load; and a control unit for controlling supply of power to the load primarily from the main battery and secondarily from the back-up battery. The device is, in particular, a single bay, stand alone parking meter. In the event that the main battery runs low, the control unit is configured to supply power to the load from both the main battery and the back-up battery or only from the back-up battery. The back-up battery is easily replaceable, and the power supply unit has a bay, with connectors for receiving the back-up battery. The main battery is charged from solar panels. A communication device is provided to communicate status messages wirelessly to a control system.

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1. A power supply unit for supplying power to a parking meter, the power supply unit including a rechargeable, main battery;a charging arrangement for charging the main battery;a set of connectors for a back-up battery;a wireless communication device for communicating a status message regarding the

1. A power supply unit for supplying power to a parking meter, the power supply unit including a rechargeable, main battery;a charging arrangement for charging the main battery;a set of connectors for a back-up battery;a wireless communication device for communicating a status message regarding the state of the main battery and the state of the back-up battery to a control system external to the parking meter;a set of load terminals for connection to a load;a control unit for controlling supply of power to the load primarily from the main battery and secondarily from the back-up battery; anda housing that encloses the main battery, the back-up battery, the control unit, and the wireless communication device;wherein the housing includes the load terminals and is received within the parking meter. 2. A power supply unit for supplying power to a device as claimed in claim 1, in which the control unit is configured to supply power, in use, to the load from both the main battery and the back-up battery when the main battery is discharged to a predetermined extent. 3. A power supply unit for supplying power to a device as claimed in claim 1, in which the control unit is configured to supply power, in use, to the load from only the back-up battery when the main battery is discharged to a predetermined extent. 4. A power supply unit for supplying power to a device as claimed in claim 1, in which the back-up battery is replaceable. 5. A power supply unit for supplying power to a device as claimed in claim 4, which has a bay for receiving the back-up battery and the set of connectors are located in the bay. 6. A power supply unit for supplying power to a device as claimed in claim 1, in which the status message is selected from the group consisting of a notification that the main battery has been insufficiently recharged and a notification that power is being supplied from the back-up battery. 7. A power supply unit for supplying power to a device as claimed in claim 1, in which the communication device includes a cellular telephone module. 8. A power supply unit for supplying power to a device as claimed in claim 1, which includes a solar panel terminal for connecting the charging arrangement to a solar panel. 9. A device which has a power supply unit for supplying power to it as claimed in claim 8, which has a solar panel that is connected to the solar panel terminal. 10. A power supply unit for supplying power to a device as claimed in claim 8, wherein the power supply unit further includes a diode coupled to the solar panel to prevent reverse current from flowing into the solar panel. 11. A power supply unit for supplying power to a device as claimed in claim 1, in which the nominal supply voltage of the back-up battery is slightly greater than that of the main battery. 12. A power supply unit for supplying power to a device as claimed in claim 1, in which the back-up battery is non-rechargeable. 13. A device as claimed in claim 1, which is a single bay parking meter. 14. A power supply unit for supplying power to a device as claimed in claim 1, wherein the communication device is configured to communicate battery status information to an external device over a telecommunications network. 15. A power supply unit as claimed in claim 1, wherein the power supply unit further includes: a first capacitor connected in parallel with the main battery to assist the main battery during peak power demand to supply the load with power; anda second capacitor connected in parallel with the back-up battery to assist the back-up battery during peak power demand to supply the load with power. 16. A power supply unit for supplying power to a device as claimed in claim 15, wherein the first capacitor of the power supply unit is recharged by a solar panel. 17. A parking meter comprising: a parking meter housing;a solar panel;a power supply unit for supplying electrical power, the power supply unit including a rechargeable, main battery,a charging arrangement for charging the main battery,a set of connectors for a back-up battery,a wireless communication device for communicating a status message regarding the state of the main battery and the state of the back-up battery from the power supply unit to a control system external to the parking meter,a solar panel terminal for connecting the charging arrangement to the solar panel,a set of load terminals for connection to a load,a control unit for controlling supply of power to the load primarily from the main battery and secondarily from the back-up battery, anda housing that encloses the main battery, the back-up battery, the control unit, and the wireless communication device,wherein the housing of the power supply unit includes the load terminals and is received within the parking meter housing. 18. A parking meter as claimed in claim 17, the parking meter further including a coin validation unit that receives power from the power supply unit. 19. A parking meter as claimed in claim 17, wherein the communication device is configured to communicate battery status information to an external device over a telecommunications network. 20. A parking meter as claimed in claim 17, wherein the power supply unit further includes a diode coupled to the solar panel to prevent reverse current from flowing into the solar panel. 21. A parking meter as claimed in claim 17, wherein the power supply unit further includes: a first capacitor connected in parallel with the main battery to assist the main battery during peak power demand to supply the load with power; anda second capacitor connected in parallel with the back-up battery to assist the back-up battery during peak power demand to supply the load with power. 22. A parking meter as claimed in claim 21, wherein the first capacitor of the power supply unit is recharged by the solar panel.