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Thanks all for sharing how you're making your PV arrays and antennae! ?I haven't flown a balloon yet, but I work with solar cells a lot. ?Here's how I plan to make my first balloon array:
1. ?I'll dice the cells by digging a trench with a dremel(tm) cutting wheel from the back. ?Then I'll break the cell. ?Fracture should cause fewer shorts at the pn junction than a sawed edge will. ?The pn junction is at the front of the cell.
2. ?I'll measure the I-V curve of each diced cell under simulated sunlight and reject any with a fill factor (FF) less than 70%. ?FF is Vmp * Imp / (Voc * Isc) (mp means maximum power point, the pair of I-V points that give the highest I*V product). ?I'll set the light intensity so that Isc is approximately 40 mA/cm^2 * the cell area for this test.
3. ?I agree the cells are hard to solder to. ?What seems to work for me is to apply a tiny bit of flux to the cell, then melt some solder onto my wire (I would use #28 gauge here). ?Then gently press the blobbed wire to the fluxed spot and touch it with the soldering iron until it yields, very much like what Mikael wrote. ?It's best to minimize the amount of time the solar cell is hot. ?I'll test each cell after soldering to see whether the soldering ruined it.
To gather power earlier in the morning and later in the evening, I'm thinking of making a dual array on a folded surface, shaped like a gable roof. ?I'll measure the I-V curve of each subarray in partial darkness to determine whether the dark array will absorb current from the illuminated one, and if it will, install a schottky series diode with each sub-array to prevent such losses. ?Has anyone else already gone down this road?
Awaiting U3B,
Halden NR7V

"What seems to work for me is to apply a tiny bit of flux to the cell, then melt some solder onto my wire (I would use #28 gauge here)."

Why are you using copper wire to solder to solar cells?

Easily available is thin 1/4" wide copper, tin, tin-plated copper, and
silver strips in any length. They are used by professional solar panel makers
because they are MUCH EASIER to work with than wire.

Gauges vary, but 30 gauge is easily available.

Where to find? E-bay has them in lots of sizes. I found some very good
25' rolls in Hobby Lobby. Both copper and silver (usually in the stained
glass department).

Maybe a bit more expensive than wire, but still reasonable, and , as I have learned,
much easier to work with on solar cells. They do weigh a bit more than wire, but not much.

john
AD5YE

John's suggestions are good.

Search eBay for solar cell tabbing wire, it's not expensive, is pre tinned and fluxed.

Search youtube for solar cell soldering - you will find many (many) videos on the subject.

But, always use flux, be generous, it can be cleaned off with alcohol and generally recommended is solder containing some silver and use a higher temperature for soldering than you usually would (for those with adjustable soldering irons), silver in your solder requires it and solar cells conduct heat away quite well so the higher temperature is required to reduce your soldering time and to keep enough heat where you need it to complete your joint.

cheers, Graham ve3gtc

On 9/17/2017 2:26 AM, John Backo via Groups.Io wrote:

Why are you using copper wire to solder to solar cells?
Easily available is thin 1/4" wide copper

The issue I see is not the wire, but getting the cell to take solder. Once the cell tins, anything will solder to it. Copper strips are likely made of copper wire that has been rolled flat and the thin wire is not a problem to solder to, at least that's my experience.

I'll try using some extra rosin flux carefully. I note that the flux from rosin core solder spreads out onto the surface of the cell. I hope I'm not "poisoning" the cell when this happens. I'll try to do some before/after measurements

John K5MO

>Why are you using copper wire to solder to solar cells?
I would use wire for this application because it's flexible in more directions. ?I prefer #28 over #30 because it withstands more nicks and bends. ?Ribbon is used to make encapsulated panels because it keeps everything thin and helps keep the circuit from moving around during lamination. ?Fortunately, it's also easy for automated soldering machines to handle, as John B. noted.
Cheers, Halden NR7V

Yes, you must put a schottky in series of each solar array or the one in Shadow will absorbe the current from the illuminated on.