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I'd like to make a solar powered USB charger, to charge my phone. I'd ideally like to use (a) solar panel(s) that I can 'mount' onto a canvas messenger bag, and use a pocket to hold my phone. Obviously I need to provide ~5V to charge a phone, therefore I need a solar panel that can provide at least that much voltage, which can thus be used with a 5V regulator to step the voltage down to the appropriate level.

What other criteria do I need a solar panel to meet? Is there a specific current that I need? Or at least a threshold current? What will the correlation be between the power output of the panel be and its ability to be used in this fashion? Just charging times?

In terms of my skills, I'm an Electrical Engineering major, but my knowledge is mostly academic, so I appreciate any real world advice.

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  • \$\begingroup\$ Or you could use a switching boost regulator. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 23 '13 at 23:06
  • \$\begingroup\$ USB provides 500 mA. So at a minimum you need a solar panel that can provide 5 V x 0.5 A = 2.5 W. Plus more for loss in the regulator/switching regulator. \$\endgroup\$ – Brian Carlton Jul 23 '13 at 23:34
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What is being requested is very well documented and widely available for DIY, and following Instructable links are examples of that.

In fact there are several questions on EE.stackexchange site, which are currently appearing on the right side panel, as I type this answer.

Check your favourite online/offline store and you'd find semi-flexible solar panels, in various dimensions. They claim to be flexible to 20-30 degrees, and could be quite fine for the messenger bag.

They key would be selection of a good solar panel -- since there can be some significant difference in energy efficiency between panels, i.e. the extent to which they can convert the sunlight into electrical energy.

Also note that, not all smartphones charge / charge-well with the standard USB charging, i.e. they make proprietary deviations from the standard, and can expect to be able to draw (much) more than 500mA @ 5V.

PS> I wanted to put this as a comment, but unfortunately, I still don't have enough reps to do anything like that. Surprisingly, I am allowed to post this as an answer.

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Often phone chargers (try to) draw more current, configured by a set of resistors across the data lines of the charger.

You'll definitely want a switching regulator; you should probably also have some sort of "brownout" protection so you're not supplying 3V in dim light.

Commercial products that do this nearly always charge a battery from the panels, then charge the device from that battery. This lets you soak up the sun for a longer period then quickly charge the device.

Mounting solar panels: normally they're not flexible; you can get "semi-flexible" ones but I've no experience as to how flexible they actually are. You'll want some sort of protection (polycarbonate? acrylic?).

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