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Look for a "MPPT Buck charger"
Just buy this, wire it up right, set the output voltage to 5V, and you'll be good.
Totally possible, though you would need a buck regulator, I do not recommend using a linear regulator if you can avoid it. Instead, use a switching buck regulator. You can buy them cheaply online.
Ideally, you want to look at a few of the ratings for the solar panel. It's maximum open-circuit voltage, short circuit current, and the maximum power output voltage/current rating (ideal load). For instance, if the 18V panel outputs 18V maximum, and 1.4A maximum voltage, but under a goldilocks load, it may sag to 12V, 800mA, where when you multiply the current and voltage, you get maximum power!
To elaborate on that a bit further, think of it like this. Imagine that I connect the panel to different things, and on one extreme, nothing is connected, on the other extreme the panel is short circuited. Here are a whole bunch of numbers that I so happened to make up. Notice as you go down the list what happens to voltage, current, and power.
No load:......... 18V, 0mA, 0W
tiny load:........ 15V, 400mA, 6W
small load:..... 12V, 800mA, 9.6W
medium load:. 9V, 1200mA, 10.8W
heavier load:.. 6V 1600mA, 9.6W
heavy load:.... 3V 2000mA, 6W
short circuit:... 0V, 2400mA, 0W
Notice as you go down the list one step at a time, the voltage goes down linearly (in steps of -3V each step.) and the current output goes up linearly, in steps of 400mA, and since 2 linear equations multiplied together with inverted slopes causes a parabola (umm, algebra...) the maximum point happens slab dab in the middle! Now real solar panels will not be that pretty, these numbers are just completly made up, and made to look nice and get the point across.
You'll need voltage regulator to bring the voltage down the the 5V needed. Anything from a basic and inefficient 7805 to a nice 5V switching regulator circuit will do. Assuming the panel can output as least 200mA.
You can built everything from anything but how can we give you advise based your information?
We really do our best here but the claivoynt bit is not implemented yet.
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Posted:May 5, 2015
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