Charging my cellphone with a 18V DC source

Question

I have received a 18V 2.5W photovoltaic panel and want to use it to either charge a cellphone (or even two at the same time) or power some Arduino projects. In both cases I need a 5V source, but mine is said 18V.

If I understand charging a device well, it is important that the source's voltage doesn't exceed the device charging recommendations, which are usually 5V, but the opposite is no problem.

How can I best make sure I get 5 Volts ?

Under the ideas I have in order to achieve that there are :

• Using a diode in order to subtract a constant amount of voltage util there are only 5V left, but it seems to me that's a very bad idea since if a cloud covers the sun, I would get less than the initial 5V, which is not dangerous, but unfortunate.
• Using some resistors (able to withstand 2.5W), but that would mean that I also need to know the resistance of my cellphone/Arduino project etc. in order to make a voltage divider... And on holiday I'm not very likely to have a multimeter in range... And it may be unfortunate to dissipate so much energy though a resistance.
• Using a charger bought on the market, but I assume they won't do the job since they are usually designed to be used in a range from 110V to 220V. (Secondary question : how do chargers actually convert voltage ?)
• Look for another solar panel :)

Thank you very much in advance for your answers.

Answer 1

Solutions 1 and 2 are going to waste most of the power from the solar panel as heat. What you're looking for is called a buck converter and you should easily be able to find one that steps down 10-25V to 5v.

Something like this would work, it has 4-40V input and adjustable output.

Answer 2

You can get these converters in prepackaged form : they are 12-24V to 5V converter for cars/trucks.

Note that the 18V 2.5W will only geterate 2.5W when in full sunlight and at proper angle, which means it will generate a lot less under realistic conditions.

Most phones require at least 5V 500mA which is 2.5W for slow charge, or 5V at 1-2A (5-10W) for fast charge. So the panel will not generate enough power.