As the cigarette lighter/accessory port provides a nominal 12 volt output, the car USB charger converts this voltage to the 5 volts required for the USB supply.

Also as the car 12 volt supply can vary considerably and also have spikes and surges on it as a result of the other car electrical systems. The car USB charger needs to protect against these so that anything connected to the USB port will not be damaged by this.

There are a number of points, hints and tips that can enable the best car USB charger to be bought:

• Multiple sockets: Consider what you might want to charge. How many devise are you likely to want to connect at any one time? Most car USB chargers have two outputs, and this is often enough, but if there is a possibility of more being required, then car USB chargers with four outputs are also available.
• Size:   It is worth considering the size of the car USB charger. This can be important if the 12 volt lighter / power point is located close to any of the driver controls.
• In-built lead:   Some car USB chargers come with a lead already built in as part of the assembly. Although individual requirements vary, this does not appear to be a good idea because the lead needs to have the required connector on the end.
• Power output:   Not all the outputs on the car USB charger may be the same. Check the current capability they provide. Often they provide around 2.1 or 2.4 amps dependent upon the device, but check what you need.