testing short circuit current

[RifRaf]

have tried looking around for bad results and cannot find them, but people i know have warned against it.

how safe is it to do a short circuit test on one or more solar cells in order to be able to test the short circuit current before embedding them into an array. have done some tests, without explosions, but would like to get some peoples experience with short circuiting cells or entire panels, does the voltage just drop down to a level way below max power output and cause no damage at all? or is a load required, be it just a shunt or something, in this case am assuming the analogue 20A current meter would be enough for constant current measurements

[Sunking]

When you do a Short Circuit test the voltage goes to ZERO. Solar panels are current sources. Providing your wiring is sized correctly and your meter has the capability of measuring the current, there is no safety issue measuring a single panel. Now if you are talking about measuring a whole solar array especially one for off-grid, there is a high degree of danger.

[Mike90250]

....there is no safety issue measuring a single panel. Now if you are talking about measuring a whole solar array especially one for off-grid, there is a high degree of danger.

Just get a welding rod holder and some flux rods, and you are all set :becky:

[RifRaf]

thanks, testing a few broken cells in sun all day shorted out did not seem an issue, they still produced the same power.

am trying to make panels using matched strings of cells, or more importantly, weed out any low performing cells before assembly. Each string consists of 6 cells, each cell is 6"x6" rated just over 8A per cell peak. So the strings should be about 3V 8A. As i still measure a voltage across the cells when short circuiting them am assuming the test leads and clips must still have a small resistance, or more likely not rated for the high current.

However these are some fairly consistent test results in good sun the other day. Seems to be an average of about 9.5W. peak power output from the string should be closer to 24W Max so am hoping the 10 seconds or so i am testing for will not damage anything. The cells are allowed to heat up in the sun for a good few minutes before testing, until Voc drops to a stable level, Voc appears higher for cold cells.

String1 Voc 3.37V Is 8.75A Vs 1.12V
String2 Voc 3.37V Is 8.75V Vs 1.16V
String3 Voc 3.30V Is 8.60A Vs 1.13V

should i make a new test jig with much heavier duty connectors and leads to remove more resistance? or accept the about 0.1 Ohm resistance that seems to still allow the voltage measured, or should i be testing some other way?

[Mike90250]

Your final and testing wiring needs to be able to handle the total amps, which I'll guess is about 9 amps (8.75) per string. That's pretty hefty amperage, and any small losses will reflect in voltage loss in your system. At your low volts losses may be unacceptable.

here's some on-line voltage drop calculators you can try out

http://www.electrician2.com/vd_calculator.htm

http://www.elec-toolbox.com/calculators/voltdrop.htm

http://www.nooutage.com/vdrop.htm

Solder is not a real good conductor, embedding a length of wire in solder will help lower the resistance of the solder trace.

Also why we don't recommend using soldered connectors, if a short happens, sometimes the solder in the joint heats up and blows the joint apart, and you now have a bare wire, splattered solder all over, and a general mess.

[john p]

Also why we don't recommend using soldered connectors, if a short happens, sometimes the solder in the joint heats up and blows the joint apart, and you now have a bare wire, splattered solder all over, and a general mess.How is it possible for a short to happen in the wires coming from the solar panel???
This argument about the problems of soldered joints goes on and on and on and on.And makes no sense to me never has never will.. as the advantages of crimping a wire connector properly then applying solder far outweighs the so called disadvantages of the claim that connectors somehow heat up then explode causing death and destruction over the whole suburb I have as yet to see,, Sure ive only been doing the crimp and solder for only 50 years so mabe when ive done it longer will see the reasons not to do it.

[john p]

And what is the proposal for joining the tabs on individual solar cells if you dont solder them??? 2"nails?? arc welding them??

[Mike90250]

.... Solder is not a real good conductor, embedding a length of wire in solder will help lower the resistance of the solder trace.

Also why we don't recommend using soldered connectors, if a short happens, sometimes the solder in the joint heats up and blows the joint apart, and you now have a bare wire, splattered solder all over, and a general mess.

Context John. Context. We are/were speaking of soldered cell contacts and metal traces. <P>

I then started a new paragraph. New idea. Launching off the thought that solder is higher resistance than copper. That even a well soldered joint has higher resistance, and can and WILL blow apart with a large current surge. The explosive action coming from both the solder vaporizing, and any residual flux wicked up in the stranded wire. (where it cannot be cleaned out)

NEC does not allow soldered power connectors. MC3 & 4 connectors are all factory crimped.

And very few folks can get a decent gas tight crimp without a powered crimper. Squeeze type and hammer/anvil types are unreliable unless a skilled operator does them.
And soldering after a good crimp, just wicks solder into the stranded wire area, making a stiff spot, followed by a flex spot, and without good strain relief, the wire will fail at the end of the solder, from stress flexing being focused on the on spot.

So, if you make gas tight crimps, solder the wire cleanly, stress relief and stabilize the conductor so flexing happens elsewhere, a soldered/crimped connection is OK. Leave out any of the precautions, and you have a failure waiting to happen.

And stop picking apart sentences. We are trying to safely work with the lowest common denominator, so they can build not too unsafe systems. Suggesting exploding solder connections are hogwash can hurt someone who believes you. As a moderator, I'm not in a good position to give dangerous advice.

Some links about this endless debate:

http://continuouswave.com/ubb/Forum6/HTML/000679.html

http://www.sailnet.com/forums/gear-maintenance/40595-crimping-versus-soldering.html

http://fsae.com/eve/forums/a/tpc/f/125607348/m/53010954411

http://forums.qrz.com/archive/index.php/t-188969.html

a extracted comment:
My opinion is this:

Best choice is to buy the high $$$$ crimping tool for every connector you use, and use no solder.

Good choice is to crimp each connector with the $10 crimp tool, and solder the connection, using solder sparingly.

Worst choice is to use the $10 crimp tool alone.

[john p]

Im sorry if you feel offended by me about your "good advice"" but I stand by my saying a good crimped and then soldered conector is supperior to just a crimped wire connector ..You do the following do one crimped connector on one wire and on another a crimped and soldered connector then leave them both out in the elements for 6 months then measure the resistance under a electrical load of each one... ok .then tell me the result..
And soldering after a good crimp, just wicks solder into the stranded wire area, making a stiff spot, followed by a flex spot, and without good strain relief, the wire will fail at the end of the solder, from stress flexing being focused on the on spot.Tell me how the wire fails when the wires and connectors are sitting in a roof space or under a panel or clipped down a wall?? I believe ALL SENSIBLE INSTALLERS install wires and connectors this way. They are not left out hanging loose between buildings.
And you forgot to telll me how a short happens to soldered connectors but doesnt happen to crimped connectors????
Your argument is same as ones I see so many times about using solid buss bar between batteries , The reason given that with temp changes the batteries are going to move so far the connections will come loose .. its in theory only NOT IN PRACTICE..

[RifRaf]

thanks for the tips. it appears true that the closer you get to a true short the lower the voltage, therefore the power is low, and that makes it safe. After upgrading the test leads, ammeter connection and tab connectors to above 20 amps was able to get far better results, with the voltage far closer to 0. is rather amazing, i still don't understand it but i understand ohms law and if the cells internal resistance is really that low when energized must be true. The voltage when shorted has dropped to below 100mV now for a string of 6 cells (was over 1V before), while showing the max current rating, will chart more results on weekend if sun stays around

am hoping from the 30 second or so test that i wish to perform to ensure that each string of 6 cells has maximum Open Circuit Voltage, and when shorted Minimal Voltage and Maximum current, this should ensure each string has no bad solder joints or weak tabbing and thus suitable for encapsulating into a panel? plan to test each side of the string by itself and also joined together at the end as if bus wire were present

[Sunking]

it appears true that the closer you get to a true short the lower the voltage, therefore the power is low, and that makes it safe.

Voltage = I x R, P = I x I x R. So if R = 0 then voltage and power have to = 0 because anything multiplied by 0 = 0

[Sunking]

NEC does not allow soldered power connectors.

Not quite accurate.

NEC Article 110.1 Scope. This article covers general requirements for the examination and approval, installation and use, access to and spaces about electrical conductors and equipment; enclosures intended for personnel entry.

Which brings us to Article 110.13 and 110.14 which specifies Cooling Equipment.

110.14 Electrical Connections. Because of different characteristics
of dissimilar metals, devices such as pressure
terminal or pressure splicing connectors and soldering lugs
shall be identified for the material of the conductor and
shall be properly installed and used. Conductors of dissimilar
metals shall not be intermixed in a terminal or splicing
connector where physical contact occurs between dissimilar
conductors (such as copper and aluminum, copper and
copper-clad aluminum, or aluminum and copper-clad aluminum),
unless the device is identified for the purpose and
conditions of use. Materials such as solder, fluxes, inhibitors,
and compounds, where employed, shall be suitable for
the use and shall be of a type that will not adversely affect
the conductors, installation, or equipment.

FPN: Many terminations and equipment are marked with a
tightening torque.

(A) Terminals. Connection of conductors to terminal
parts shall ensure a thoroughly good connection without
damaging the conductors and shall be made by means of
pressure connectors (including set-screw type), solder
lugs, or splices to flexible leads. Connection by means of
wire-binding screws or studs and nuts that have upturned
lugs or the equivalent shall be permitted for 10 AWG or
smaller conductors.

Terminals for more than one conductor and terminals
used to connect aluminum shall be so identified.
(B) Splices. Conductors shall be spliced or joined with
splicing devices identified for the use or by brazing, welding,
or soldering with a fusible metal or alloy. Soldered
splices shall first be spliced or joined so as to be mechanically
and electrically secure without solder and then be
soldered. All splices and joints and the free ends of conductors
shall be covered with an insulation equivalent to that of
the conductors or with an insulating device identified for
the purpose.

Wire connectors or splicing means installed on conductors
for direct burial shall be listed for such use.

Ok that has nothing to do with what we are discussing, but here is the section that applies. It is from Article 230 which covers Services.

230.81 Connection to Terminals. The service conductors
shall be connected to the service disconnecting means by
pressure connectors, clamps, or other approved means.
Connections that depend on solder shall not be used.

Problem is the panels themselves are not covered by NEC. That would fall under an Approval agency like UL.

[john p]

I read this to mean that if the connection is mechanically sound(good crimping) then adding the solder is acceptable ?/ I believe adding the solder does a hell of a lot to stop the ends of the wires corroding inside the connector. I still also believe people that make statements that the wires are going to fail because of vibration or movement are doing so with no evidence to prove that, The reason they have no evidence is because it just isnt going to happen.. They were told this by somebody who told them who told them and that person was a used car salesmen.
example wires from solar panels are first going to be clipped under the panels then fed either in conduit or into the roof space then clipped down the wall to the charge controller..Now tell me how all this cable movement is going to cause the wires to break because they were crimped AND SOLDERED ????? aND HOW ARE SHORT CIRCUITS GOING TO OCCUR BECAUSE THEY ARE NOW SOLDERED???? ..
After nearly 50 years of crimping and soldering im still waiting to see the first "exploding with spatter and short circuiting " connector..
For the home panel maker he /she is unlikely to have a proper power crimper but can still make a "fairly good" connector with other means.. I n fact a good pair of vice grips can do a damm good job on cables os #4 or less as long as you REALLY squeeze them hard on that last go. then if solder added inside the connector a very acceptable connection will result.. I know as have done this method thousands of time so have many others ive worked with.

[Mike90250]

......
For the home panel maker he /she is unlikely to have a proper power crimper but can still make a "fairly good" connector with other means.. I n fact a good pair of vice grips can do a damm good job on cables os #4 or less as long as you REALLY squeeze them hard on that last go. ....

This has to be pure BS. Or your #4 crimp terminals are made from foil. No vice-grip I've seen has a decent jaw pattern for gas-tight crimps, which are what's needed for long life. Or else your hands are really strong, and I beg your pardon sir.

And John Wiles solar code corner on connections http://photovoltaics.sandia.gov/docs..._Corner_96.pdf

[Sunking]

I read this to mean that if the connection is mechanically sound(good crimping) then adding the solder is acceptable ?

No John quite the contrary. Only place solder is permitted in the US NEC code is with cooling equipment and Knob & Tube wiring which is an antiquated wiring method from pre-dating the early 60's.

In all instances solder is permitted you will see the following statement:

(E) Solder. Connections depending solely on solder shall
not be used.

What this refers too is mechanically solder is very weak and support and strain relief much be provided.

Fact is solder connections are inferior to mechanical permanent compression connections in every aspect form connection resistance to mechanical strength. Now with that said, workmanship and materials play a huge role. One has to use compression connectors listed for the intended use, on the type and size of wire being used on, following manufactures instruction with approved tooling listed for the device. Using a hammer, vice, or vice grips with solder are not approved. MacGyver is not on any code making panel

Folks may not like that answer, but those are the facts here in the USA. People have lost property and lives learning the lessons.