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I found a small solar panel with this spec:

Power: 1.25W Voltage: 5V ma:250

this is an iphone 6 batery:

Current: 1809 mA Power: 6.91 Whr Voltage: 3.82 V

Would it work if I just modify an iphone cable by cutting the usb part and solder the wires in the solar panel positive and negative and then plunging the lightning part in the iphone? And then how long would it take to charge?

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closed as off-topic by Chris Stratton, Voltage Spike, Wesley Lee, Nick Alexeev Jul 20 '17 at 6:07

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Chris Stratton, Voltage Spike, Wesley Lee, Nick Alexeev
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Nope, it wont work, solar panels are variable sources. Secondly, please read the guildlines on posting in the help center before asking questions. Buy a solar usb charger, its cheaper than rolling your own \$\endgroup\$ – Voltage Spike Jul 20 '17 at 5:28
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This unfortunately wouldn't work as the iPhone, once connected, will draw down the voltage of the panel and halt any charging activity. This is because a solar panel is electrically a current source, not a voltage source, and so cannot maintain its output voltage under heavy load.

When the iPhone is connected to a power source, it will attempt to draw current at 5 V from the source. Most iPhone chargers can supply more than 1 A current, and generic standard chargers will supply at least 500 mA. The iPhone, once connected, will draw all 250 mA that the panel can supply (it is attempting to draw even more current). When this happens, the solar panel voltage will drop out and charging will stop. This is because a solar panel can only operate on its I-V curve. For more info on the physics of why this happens, read up on the theory of solar cells.

generic solar panel I-V curve

When the panel current is at its maximum (250 mA), the voltage is 0 V. This is also known as the short-circuit current (Isc) operating condition. You can try this for yourself by connecting the positive and negative leads of the solar panel together and measuring the voltage between them. What you'd see on your iPhone setup is the charging starting, then stopping, then starting, then stopping ... resulting in no charging. To make this work, you need a larger panel plus some other components. There are a lot of Instructables that show you how to do this.

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I would avoid just cutting the cable and attaching it to your panel. Unless your panel has built-in regulation so that it will ALWAYS output 5v, do not plug your phone right into it.

Solar panels tend to be rather finicky, and their output current and voltage tends to fluctuate wildly with how the sun hits them, how cloudy it is, rain, etc. Depending on the panel, you could see open circuit voltages upwards of 20v!

What you should do is purchase a low drop-out regular like This and use it inline with your phone so that it gets a constant 5v.

Otherwise, you risk damaging the device.

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  • \$\begingroup\$ Oh I see. So with that would it actually work? Would it charge? and how long would it take? I live in canada so I can't buy that item.. Would this work? dhgate.com/product/wholesale-50pcs-lm7805-l7805-7805-voltage/… \$\endgroup\$ – rick Jul 20 '17 at 3:49
  • \$\begingroup\$ Yes! The rate of charge all depends on how much sun you get. If your panel is rated for 250ma, then your phone would probably charge at 1/3 of the rate it would while connected to a regular charger. Try ordering from digikey.com They ship to canada and allow small orders. Since you probably only need one. \$\endgroup\$ – Bradyn Claycomb Jul 20 '17 at 3:53
  • \$\begingroup\$ This is probably not a productive course of action. Fundamentally, it's based on too many guesses - that the solar panel yields raw output rather than having an internal voltage converter, that the iphone will accept a lower than normal charge rate, etc. Questions on stringing together undocumented products like this are generally considered off-topic here, because they rely on such guesswork rather than applying engineering principles to available data. \$\endgroup\$ – Chris Stratton Jul 20 '17 at 3:57
  • \$\begingroup\$ But now that i'm thinking about it, I think you will have better luck with a LM7805 regular. It is probably one of the most common electrical components out there among hobbyists. digikey.com/product-detail/en/fairchild-on-semiconductor/… \$\endgroup\$ – Bradyn Claycomb Jul 20 '17 at 3:57
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    \$\begingroup\$ @rick - you should buy a solar panel specifically designed for charging phones. Or else you need one with actual engineering specifications, and you need to know the minimum current the iphone will accept for charge. \$\endgroup\$ – Chris Stratton Jul 20 '17 at 3:58

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