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Topic: Solar Charger (Read 714 times) previous topic - next topic


BTW have you measured whats on pin 5 of the connector.

Its part of the connector standard and iv noticed some android leads do not play nice with apple.

Charger themselves are interchangable.


Bears repeating.

As you draw more current from it the voltage will drop. You can measure it at 4.7V in bright sunlight with no load attached, but as soon as you attach any load that voltage will drop. Try attaching a small automotive (12V) light bulb and see what the voltage reads.

4.2V is the fully charged voltage for a LiPo. 4.7V is the exploding or on fire voltage.

Apologies yes, i was making allowance for a series diode and was not clear.

But he said it works with a kindle, 5 pin connector.
I assume he uses a lighning adapter.
Another possible problem even if pin 5 is configured correctly is, under load can the solar cell supply sufficient volts to power the authentication chip in the adapter.


See post #11.
How did you measure the PV output.

Thanks ..Tom........ :)
Everything runs on smoke, let the smoke out, it stops running....

jack wp

Jul 12, 2015, 04:26 pm Last Edit: Jul 12, 2015, 04:41 pm by jack wp
using two 500mA 5v solar panels,

Solar panel ratings are not to be relied on. You must test them with a VOM to find out what voltage and current is really being provided while driving your load, and the sun where you want it.

I suspect the 500ma rating is with maximum sun, and into a short circuit (less than .5 volts).
I suspect the 5V is with no load and a nominal voltage (meaning it could go up higher around 6 or 7v).

I suspect the panels output current at 5V would be lots less than 500ma (maybe 90ma).

Was there a rating in watts ?
If you want to provide 1000ma (1 amp) at 5 volts into your load, that would require 5X1= 5 watts. Add another 3 watts for good measure (sun not exact, efficiency of regulators etc.), then you need 8 watts. I suspect your solar panels will not provide 8 watts (I could be wrong ).


Jul 15, 2015, 12:35 am Last Edit: Jul 15, 2015, 12:40 am by CantSayIHave
The ratings for the panels are 5v, 500mA, and 2.5W.

This is where I got my panels from. As you can see in customer reviews, most people are fairly happy with it.

You can also see it outputs much higher than 90mA when running at 5v. For these reasons, I'm fairly confident I can get this to work.

If I wire them in series and use a switching regulator, is there any way I can boost my current from ~500mA to ~1A?


As you can also see from the customer reviews, many people do not get 500 mA or 2.5 watts from the panels, even in full sun.

PV cells don't work well with switching regulators, so it is very unlikely that you will get 1 A from the series combination.


Jul 15, 2015, 09:56 am Last Edit: Jul 15, 2015, 10:16 am by Boardburner2
Also they vary from panel to panel, to be sure you have to measure YOUR OWN under load.

I purchased four 10 watt panels some years ago.

Their measured outputs were between 2 and 6 watts.

They were very cheap though , probably seconds so i was not expecting much.

One of those reviews says it will charge an android but not an iphone !.

Read my previous post on authentication chip.
I also notice they claim a conversion rate of 17 %.

I plain do not believe that.


Jul 15, 2015, 11:10 am Last Edit: Jul 15, 2015, 12:24 pm by Boardburner2
Here is a 2.4W solar cell


They specify test conditions and I believe that spec.

It is nearly four times bigger than yours.

I would not expect more than a watt from the panels you have.

Do these solar panels have diodes built in for 1 way charging or can they feedback? 


  The cells do not appear to have any external components. They do not have any wires either. The front surface is coated with a thin rigid clear material. The rear surface has solder terminals. I have installed them in an outdoor application about a month ago (july 2014) and they are still functioning as expected. Each of my cells is driving a network of 4 microprocessors. In full sun typically they output about 200 ma @5.1 volts. With sun behind a dense cloud, the output current drops to about 60 ma @ 2.2 volts.  see less   
  john king answered on August 21, 2014   

  See more answers (1)

jack wp

Seems like the solar panel ratings are not to be depended on.
If you really want 1000 ma (1 amp) you need a bigger boat. (Jaws).


Jul 17, 2015, 05:52 am Last Edit: Jul 20, 2015, 09:21 am by lemming
The voltage rating on solar panels is usually that attained at rated current in full sunlight. If you  test the voltage, open circuit in full sunlight you will get a voltage well above the rated voltage. My 12 volt 10w panels produce 21 volts open circuit, which is pretty normal. The voltage drops off rapidly as the load increases. The obvious answer lies in putting a regulator on the output to get a steady voltage. A switched mode regulator should be used as linear ones would be too inefficient. The exception may be if the solar panel output voltage is too close to the voltage required by the device to be charged. In this case a LDO regulator or better still a SEPIC type unit (buck/boost) could be used. (e.g. http://www.dx.com/p/zndiy-bry-dc-dc-automatic-voltage-stabilizer-regulator-module-w-solar-cell-331794   or  http://www.dx.com/p/dc-dc-converter-auto-step-up-step-down-solar-power-supply-module-red-151576

However, the above is fairly moot, unless your current draw is very small, as the panel voltage drops off very very rapidly once the current draw approaches the short circuit current rating of the panel. There is an excellent article on the phenomenon on Adafruit:  https://learn.adafruit.com/usb-dc-and-solar-lipoly-charger/design-notes with some explanatory graphs.

Consequently, if you are drawing more than a small amount of current you will get rapid oscillations as the voltage drops; regulator drops out; voltage rises; regulator cuts in; load rises; voltage drops; regulator drops out, etc.

To get the best efficiency (or merely to get it to work with small panels like yours) you need something to control the current draw dynamically (and quickly) to deal with variations such as clouds passing over and to optimise the panels output automatically for the amount of light available even in static situations, e.g. a dull day. (Again, see article above). The most power you can get out of your panel is when you draw the maximum current without sending the voltage below somewhere around 4 volts, on your panel, and vary this current draw to ensure the voltage hovers around this 'sweet spot'.  Adafruit achieve this using the Microchip MCP73871 in a product http://www.adafruit.com/product/390. (I have no affiliation with Adafruit. There is a similar product on Sparkfun too).

The voltage output by the unit may still be below 5 volts but you could boost this with Crossroads suggestion (or ones like the two buck/boost units that I suggested above), attaching this to the load terminal of the solar regulator unit. You don't have to have a LiPo connected to the regulator unit to do this.

This way you know the setup will work in sub optimal lighting and will optimise your charging for any conditions.

Needless to say, you can't expect to draw more power out of the setup than the the panel can provide. The above will optimise your solar charging but cannot produce something for nothing.   




So I visited jremington's link (Thanks!) about data line voltages, and it proved very useful.

I bought resistors for this circuit -

and it's actually working fine.

The adafruit page mentioned that the circuit I'm using should only accept about 500mA, and this seems right, as it's charging near half speed. I'm thinking about looking into this circuit -

which might give higher current.

Also, I replaced the two separate diodes with one main diode, raising the unloaded circuit to 5.8 volts, so the solar panels appear to be working as advertised.

My question now is - with the current circuit, the + and - leads of the charger are connected, albeit through resistors. Will this damage the panels when not connected to a device?

Thanks all for the advice!

jack wp

Looking at the last two schematics, I can't determine where the solar panels ar.

But for your question, it is very hard to damage solar panels without mechanical force.

I have 22 panels that have been working fine for over 20 years. I had one that was damaged, because I ran over it with the truck.. Really!


I should imagine a truck like that is very useful in traffic.
Left , right , nah over  :o

Interesting though that the supposed authentication chip turns out to be equivalent of a few resistors.

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