Solar cells Ebay Wiring question?

[andyman5002]

Hi all, Im in the UK and I might be ordering some solar cells of ebay, the ones you guys all use to build these amazing panels. But what I can't figure out is how you get the the volts and amps you get?

The cells are advertised as :

    * Updated Specs Peak Power--1.6 Watts

    * Current (max power)--3.5 amps

    * Voltage (max power) .5 V

    * Open Circuit Volts--.6 volts

    * Short Circuit Current--3.85 amps

Now you get 20 of these in one order, so what panel could I make with this?. I want to make a 12V system.

Now acording to me, to get 12 V you would need 24 cells wired in series??

Any help with the maths would be great

Thanks alot Andy

[craig110]

Hi Andy,

The wiring can sound intimidating, but it is pretty straightforward if you keep this in mind: Series means add the voltages and parallel means add the amperages.  So yes, putting 24 of those cells in series will give you 12v at the amperage of each one (3.5A).  Putting 24 of those in parallel (not that you asked about this) would give you 84A (24 * 3.5) at the voltage of each one (.5v).  Note that either way you get the same total power, 42 watts, which makes sense as you are connecting 24 items that each produce 1.75 watts.

Some people get confused trying to put some in parallel and some in series.  My wording of the general rule of thumb is "fill the box" in that it doesn't matter how wide the box is (how many in parallel) or how deep the box is (how many in series), just make sure the box is filled to capacity.  So, as an example, if you wanted 35A at 5 volts you would need a "box" that is 10 wide (to add the amperages to 35) and 10 deep (to add to 5 volts) so you would need a total of 100 cells to fill this sized box and give you 35A at 5v.  What happens if you don't fill the box?  Bad things.  ;-)  The cells will still produce power, but the output that you get won't be easy to calculate and it is not an efficient way of using the cells.  Some people think that taking 19 of your cells and putting 10 in parallel (to act like one big cell with high amps) and then the other 9 in series after it to boost the volts would give 35A at 5v.  It doesn't work that way since 35A worth of electrons can't flow through a 3.5A cell with each electron getting a voltage boost.  Just remember to "fill the box" and you'll get the output you were hoping to get.

One side question for you - is the desire to get 12v to charge a 12v battery or is it to run something that takes 12v?  If the latter, ok, but if it is to charge the battery then you get into the issue that you need a higher voltage to push electrons into the battery.  I won't go into this as it is covered in plenty of other places in this site and I don't know that that is what you are planning on doing.

Craig  

[andyman5002]

Hi craig, thanks for the quick and informative reply. Basically I want to charge batteries and then run an inverter off it to power light bulbs and maybe other things.

So I guess I would need more than one set of 20 panels. So If I had 40 panels 27 of which are needed for 13.5V, I cant really use the rest can I because it wont fill the box. So id have a 47.25W panel? Am I correct? But what can I do with the other panels?

Regards Andy

[andyman5002]

Or should I just up the voltage to say 18.5V thus increasing the power aswell, and using more panels?

Andy

[craig110]

Every time someone asks what voltage they should pick to charge their batteries, I'm reminded of what some Soviet Union naval engineers answered when, after years had elapsed, of course, they were asked why their first ballistic missile boats had the identical configuration (16 missiles aft of the conning tower) of the early US missile boats.  Their answer?  "We knew the Americans had spent millions, if not billions, researching the most efficient submarine configuration, so why spend all that research money when we can just copy their answer?"  (Not the exact wording, sad to say.)  How does this apply to designing a solar panel?  The commercial "nominal" 12v panels have enough cells to generally put out between 18-21 volts open circuit.  This gives enough volts to charge 12v batteries (which, incidently, can go up to 14ish volts at full charge depending upon the type of battery) after subtracting out the voltage drop due to the load, the voltage drop due to the losses incurred in normal wiring runs, etc., etc.  You could deviate from the industry's general answer of 18-21v OC if you know the exact details of your situation, or you can take the old Soviet Navy approach and just "copy their answer" knowing that the industry has researched this and wouldn't put extra cells into a panel if they knew they would be wasted.  :-)

Craig

[craig110]

Hi Andy,

(By the way, calling the individual items "panels" can be confusing as a collection of individual solar cells, which you are thinking of buying, wired together make up one panel.)

I answered the voltage question below, but yes, you're getting the idea of "filling the box."  If you had to buy them in batches of 20 and 27 would fill your box, the other cells should either be used elsewhere, kept for future projects (my favorite - and I have the cluttered storage areas to prove it ;-), or sold to someone else.  If you did put those cells into a 3x9 configuration, yes, you'd have a total of 47.25 watts of production.  Keeping with your hypothetical of wanting 13.5 volts for simplicity, you could always use 9 of the extra cells to add another row and end up with a 4x9 configuration.  That would up your amps and just leave 4 cells unused.

But, lets go back to the panel output watts to describe one more thing you need to be aware of.  Watts out of the panel is not the same as watts into the batteries.  Lets go way down to the microscope level for an instance and pretend that we're watching a 16v electron that is going into a battery that is currently at 12v.  What happens?  That 16v electron finds a "spot" that needs charging (using inelegant terms as the technology, and thus the particular thing being charged, varies from battery type to battery type), and gives that "spot" 12v to charge.  Does the original electron end up still having 4 volts?  Nope.  The 4 extra volts were converted to (boo, hiss) heat.  So, the actual power being absorbed by the battery at any instant in time is the number of electrons coming into it, measured by the amperage, times the present voltage of the battery.  Instead of the 3x9 configuration (which would only produce 4.5 volts and thus won't charge the batteries at all), lets pretend it is an outrageous 3x90 configuration putting out 45v at 10.5A or 472.5 watts.  Connect that to the batteries and what you end up with is 10.5 amps worth of electrons flowing into the batteries and each one charging the battery at only 12v.  That means this hypothetical panel would have produced 472.5 watts and yet only 126 watts of power (10.5A / 12v) was being absorbed by the batteries.  That is an efficiency that is somewhat less than desirable.  If you want to learn more about this, search for "MPPT" on this board or on google and you'll learn about a technology in newer charge controllers that would, in this example, sense that the battery only wanted 12v and would convert the 472.5 watts into 12v at nearly 40A before applying the juice to the batteries.

Craig

[Volvo farmer]

In other words, since nobody's said it yet... Use 36 cells per panel. Order 2 sets of 20 and this will leave you with four leftovers and since these cells are imperfect, you pick the four worst ones to leave out. I was getting about 19V open circuit and 3A short circuit on a 36 cell string. I was figuring on about 50W per string with losses and imperfect cells. These cells are Evergreen culls, meaning that they weren't good enough to go into their panels. Evergreen sells 102, 110, and 120 watt versions of their panels made with these cells. Here's the specs on their panels. I've never seen over 20V open circuit with my panels and evergreen advertises over 21V.

http://shop.altenergystore.com/itemdesc~product~Ec120%2C+120w%2C+12v%2C+Mc+White+Background~ic~EVE12
0%2DEC120GLK~eq~~Tp~.htm

[andyman5002]

OK thanks guys for all your help, youve answered all my questions. Im going to go and do the calculations and see how mcuh this project comes to.

Thanks for all your help