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I've been looking around for some solar panels for a little homemade solar phone charger project. I recently found a portable phone charger project from a student at Cal Poly which I'm hoping I can add solar panels too instead of using batteries. I'm wondering two things. First, can I replace the batteries in this project with solar panels? Second, if so, what panels should I use?

I've been looking on Ebay and the Google and there are a lot of options. I'm hoping someone might be able to give me a more direct answer.

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That charger takes any voltage between about 2V and less than 5V and boosts it to 5V.

You could use that circuit with a 2V to say 4.5V solar panel BUT you can instead just use a solar panel directly.

A solar panel with full sun loaded voltage of 6V or more will charge a USB cellphone either directly or with the addition of a series diode. (Diode needed ONLY if cellphone battery voltage appears on charging input. )

For 'safety' you should also connect a 5V1 zener-diode (5.1 Volt) across the solar panel output so that Vout can never be > 5.1V.
Zener Wattage = Wmp panel = max panel power in full sun.

Charge rate is related to panel Wattage.
Panel peak watts in full sun = Wmp.
The panel wattage for maximum charge needs be no more than :

Vpanel_loaded_at_full_sun / Ah_of_cellphone_battery

eg a panel with Vmp=6V and charging a 1000 mAh = 1 Ah cellphone battery need have a Wattage of no more than 6V/1Ah = 6 Watts. This would charge the battery at maximum possible rate. Higher wattage panels will heat the zener and lower wattage panels take longer to charge.

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