Power supply unit

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; and

a 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; and

a 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, and

a 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; and

a 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.