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Thread: testing short circuit current

  1. #1
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    Default testing short circuit current

    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

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    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.
    MSEE, PE

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    Quote Originally Posted by Sunking View Post
    ....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:

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    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?

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    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.

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    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.

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

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    Quote Originally Posted by Mike90250 View Post
    .... 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-m...soldering.html
    http://fsae.com/eve/forums/a/tpc/f/1.../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.
    Last edited by Mike90250; 11-08-2010 at 05:13 PM.

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    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..

  10. #10
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    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

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