Safe way to fault-test voltage of panels IN SERIES?

newestnewbienewestnewbie Registered Users Posts: 6 ✭✭
When you google you find lots on
- voltage testing of an individual panel
- warnings not to test multiple connected panels
- one video describes testing panels connected parallel.

Here, and nowhere though, I found a way to SAFELY test voltage of panels IN SERIES, does any electrician know?

So I have just 3 panels, connected in series (for higher voltage serving a 24V offgrid battery bank), and all has been working flawless for 2 years. Despite that, from the start I noticed that when I lifted the panels more horizontally towards the sun (we have an ingenious flexible mounting on the rear of a mobile home) then there appeared to be a loose connection somewhere.
Never found where, so I just left them in a bit lower position, and all has been fine.

Now however when I lifted them to a higher position, I somehow caused a permanent disconnect in the wiring/connections.
Measured each panel individually, all 3 are still working well: 19.6V
Measure at the end of the cables inside the tiny house however, the 3 combined now only bring in a few millivolts.
Cables look intact.

I am wondering how to find with my multimeter the fault in the connectors between the panels. What is a safe way, where may I touch the probes on one end of the series and on the other end of the series, anyone knows?


  • mcgivormcgivor Solar Expert Posts: 2,408 ✭✭✭✭✭
    There is really no way to find the fault described with a multimeter if in series with MC4 connectors, the only way would be divide and conquer. Disconnect all loads, or cover the panels with a blanket, to prevent arcing get a set of MC4 extention cables, then bypass each panel one at a time reconnect load to see what output you have. Never disconnect under load, the appropriate tool to separate the  MC4 connectors is required to prevent damage of the connectors. Thinking logically the most obvious places a fault would occur is at termination points, connectors, screw terminals or where movement occurs. Keeping in mind faults sometimes only occur when a load is present and current is flowing, voltage by itself is not a reliable way to test a circuit.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • newestnewbienewestnewbie Registered Users Posts: 6 ✭✭
    I managed to sketch it out, for easier understanding :)

  • EstragonEstragon Registered Users Posts: 3,021 ✭✭✭✭✭
    Just to reinforce @mcgivor 's point on testing with load (as opposed to measuring open circuit voltage)...

    I had a fault in generator wiring that drove me nuts for several days. Wiring all tested fine for continuity with multimeter, but the genny wouldn't start. Only after I tested the circuits with load (I used a pair of 6v lantern batteries and an automotive filament bulb connected with clips to each end of the segment or circuit to be tested) did I eventually find the fault.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,132 ✭✭✭✭
    And then there are intermittents in the panels. I hope the OP has not been lowered into this category. It is the worst...
    But as above, the only way you will probably find the bad one is under load.
    Look at each panel face for discoloration, ala BP Solar.

    Hope these are not BP ?

    Talk about intermittents....Almost all of the BP panels got them about the same time. I spent days on a neighbors BP's.....
    The problem was easy after that. Replaced them all !

    The only satisfaction I got on this job was when BP was found guilty in the Gulf oil spill. ;)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 28,155 admin
    If BP 175 Watt panels from 10+ years ago, about 1/2 of my panels have "coffee/brown" stain like color (almost looked like a dusting of rus) under the glass just under the J-Box on the back of the panel.

    BP did replace them under warranty (after BP got out of the solar business) a few years ago.

    If you can get/have a DC Current clamp meter (or have somebody watch the inside controller/meter), just start moving wiring around (one place at a time) until you see the meter/current jump.

    Many times, this has been the result of poorly crimped MC4 connectors (sometimes just  a gentle pull will pull the wire out of the connector).

    DC Current Clamp type DMMs (examples):

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,132 ✭✭✭✭
    These were BP 165's and they did pay. We went to another brand that I could get warranty on. I still do not like them and a movie I watch is the Deep Water Horizon with Mark Walberg. I am not that against BP (they have a ton of employees) but it can be fun to see them stumble in that movie. 

    I remember when I was shooting those BP's, I kept getting different current readings every other time I measured.....

    We had some BP's too but they got replaced with more watts per square foot for tracking. God help the guy who has them now....
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • littleharbor2littleharbor2 Solar Expert Posts: 1,103 ✭✭✭✭
    edited August 5 #8
    Those BP 170 and 175 watt panels were actually really nice well made panels EXCEPT the J-box and its lack of strain relief for the cables. All it would take to ruin an otherwise perfectly good panel would be to tug a little too much on the output cables causing the soldered connections in the box to crack and fail.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Dave AngeliniDave Angelini Solar Expert Posts: 4,132 ✭✭✭✭
    The J-box and water getting in thru the back and the big one was not much power per square foot. The power got better in most panels about that time. Not as good as Panasonic and Sunpower.   8 x 175 =1400  in the same array footprint 6 x 330 = 1980
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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