solar Iphone charger

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slurry bowl
 
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solar Iphone charger

Post by slurry bowl »

Hey forum !

You helpful people, I want to thank you for being you.

Anyway.....

I have some large solar panels waiting to go on a roof and wanted to see how many Iphones I could charge directly off the panel.

The panel is 260W with the following specs:

Voc= 37.92 Volts
Isc = 8.6 Amps
Temp coefficient for V = -.33 per deg C. (deviating off STC which is 25C)

Anyway, I know I need a buck converter and Adafruit does not sell any robust enough for this job, but Pololu does and they have this one:

https://www.pololu.com/product/2850
which will step up to 37 Volts down to 5 V and provide up to 2.5 Amps.

In bright sun, the panel will produce about 7 Amps MAX, but generally less, around 4-5 Amps.

I watched Ladada's video on Iphone charging and realize I need to set D+ and D- to either 2V or 2.8V in order to get the phone to charge. Has this been found to be true with Iphone 5 and 6?

Also, will the buck converter just get SUPER hot ?

I know by allowing the panel to warm up before plugging in, I can keep the voltage under 37V which is the max for their board.

Any thoughts?

thanks

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adafruit_support_mike
 
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Re: solar Iphone charger

Post by adafruit_support_mike »

If we take 37v @ 4.5A as moderate expected output figures, the total power will be about 167W.

If you reduced that power to 5v with perfect efficiency, you'd get about 33A.. enough to charge 66 iPhones at 500mA.

You won't get perfect efficiency, but buck converters (which reduce voltage and increase current) are pretty good these days.. 95% efficiency isn't all that unusual. Multiplying 66 by 0.95 and rounding to make the numbers look nicer, you could probably charge around 60 phones.

WRT heat, that's generated by the roughly-5% loss in efficiency, and 5% of 167W is about 8.5W.. enough to get fairly warm.

You can avoid the heat problems by distributing the loss across several buck converters though. Finding a single converter that can handle 33A would be a challenge, but finding 8 that can handle 5A isn't so hard. Each converter would only generate about 1W of heat, and would be designed to handle more.

pdh
 
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Re: solar Iphone charger

Post by pdh »

> The panel is 260W with the following specs:
>
> Voc= 37.92 Volts
> Isc = 8.6 Amps

This doesn't really affect the gist of Mike's answer, but you're misinterpreting the specs on the solar panel. Notice that 37.92 V * 8.6 A = 325.94 watts, which is way higher than your rated 260 watts. The reason is that Voc is the open-circuit voltage of the panels -- i.e. the voltage that they'll produce with no load applied. There should be another spec on the panel for Vmp, which is the voltage at maximum power -- that will be a lower number. There will also be a spec for Imp, the current at maximum power, and if you multiply Vmp by Imp you should get a number very close to 260 watts -- that's how the 260 watt spec is derived. Use the Vmp and Imp specs when computing actual usable power from the panel, not Voc and Isc.

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slurry bowl
 
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Re: solar Iphone charger

Post by slurry bowl »

great info, thanks so much.

It seems that even with the losses associated with the buck converter, I can still charge quite a few phones.

I realize the specs on the panel and have the Vmp and Imp.

My concern is that when I first plug in the buck converter, the voltage could briefly be close to Voc, which is higher than most buck converter ratings of 37V. If in extreme cold and bright sun, the Voltage when I plug it in could be around 42V.

Is there a way to prepare for this brief high voltage by introducing a capacitor in parallel with the buck. If so, how do I size it?

Has anyone found that Ladyada's resistor specs on D- and D+ work for Iphone 5 and 6?

Thanks for the input!

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slurry bowl
 
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Re: solar Iphone charger

Post by slurry bowl »

I still dont fully understand how PV panels are current limiting, but is it true that the extra amps not used if I only have 2 phones plugged in will be dissipated as heat? OR will the panel only produce as much as the circuit needs?

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adafruit_support_mike
 
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Re: solar Iphone charger

Post by adafruit_support_mike »

The concepts of 'voltage' and 'current' provide enough information to explain circuits with an external power source, but if you want to talk about power sources themselves, you need another piece.

Electromotive Force (EMF) is what makes electrons move. Current measures how many electrons are moving and how fast they're moving (it's a statistical thing, so while the electrons themselves are moving at the speed of light, the average of all their velocities is usually less than a millimeter per second). Voltage measures the difference between how fast the current *is* moving in response to the EMF and how fast it *wants* to move. Resistance (or, more generally, impedance) measures something in an electron's path that keeps the electron from moving as fast as it wants to go.

If you apply EMF to a material with no resistance, you get current without any voltage. That's approximately what happens in wire, or in the traces of a PCB.

If you apply EMF to a material with infinite resistance, you get voltage without any current. That's approximately what happens in a battery that isn't hooked up to anything.

If you apply EMF to a material whose resistance is between zero and infinity, you get some combination of voltage and current.

Current is always limited by the number of electrons that *can* move, so there's no such thing as 'unused current'. If you have a power source that can deliver 5A and connect a 1A load to it, it's structurally incorrect to ask where the other 4A go. They don't go anywhere.. you just have a difference between how much current the EMF can move and how much current actually is moving. We've already given that difference the name 'voltage'.

So.. if you have a solar panel generating enough EMF to move 5A @ 5v, and you connect a load that only allows 1A to flow at 5v, the unused energy from EMF shows up as additional voltage.

The exact relationships among a solar panel's EMF, voltage, and current are complicated, but a large amount of unused EMF only produces a small increase in the panel's output voltage. In the "wants to move 5A but only 1A is actually moving" scenario above, the panel's output voltage only rises by about 35mV.

At the atomic level, the electrons in the silicon matrix move to higher energy levels like water rising behind a dam. The exact water level strikes a balance between "how much new water is arriving?" and "how much water is the head pressure pushing through the spillway?"

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sbessone11
 
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Re: solar Iphone charger

Post by sbessone11 »

Dear friends, hope someone helps me with this: I'm trying to charge an iphone 5 and an iphone 5s with my 3,5v solar panel and I can't. I'm using a 0,9V-5V to 5V step up booster, getting a total 5V 500mA from the solar panel. I connected 2v for each usb data pins, using 51k ohm and 75k ohms resistors, but the iphones not only dont charge, but they discharge! I tried using the same booster but with a simple AA battery, that through the booster delivers 5V 700mA, but it also discharges the iphone! What may I do? Does any know how to solve this problem? Is there any additional circuit I should use? Thanks, Seb

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slurry bowl
 
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Re: solar Iphone charger

Post by slurry bowl »

If not in direct cohesive sun, the panels often put out only a small fraction of rated wattage. Check with a clamp on meter to get an amperage reading. I think you need 43K and 75K ohm resistors to get the correct voltage handshake with apple phones.

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