Disconnecting panels on camper

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mikereilly
mikereilly Registered Users Posts: 7 ✭
in Solar Beginners Corner #1
Another newbie question from me:

I have two 100w panels that I intend to connect in parallel on the roof of our camper trailer.  They are going to be removable in case we find some shade to park in and have a pair of 28ft 10 gauge wires to allow us to put them in the sun nearby.  I notice the cables and connectors I have say not to disconnect under load.  How important is this?  It's only a 12v (ok, probably up to 18v) system, so like a charged 12v battery, what's the risk?  MC4 connectors and all joins.  Do I have to fully cover the panels every time I want to move them?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    edited July 2019 #2
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    MC4 connectors are actually quite fragile (as far as connectors go)... They are high pressure flash gold plated--Which because of the high contact pressure can be wiped off in a few dozen cycles (I forgot the exact specification).

    Also, they usually require a tool to unlock. This can be an issue if somebody forgets to put the panels away and starts driving off... Using a simple plug and receptacle (like a small 240 VAC plug so it does not get mixed up with your 120 VAC cords), will carry the current and pull apart if driven away or tripped over.

    The other issue is voltage drop... 12 volt panels and wiring are not very fond of long runs (unless you use very heavy copper wire)... A quick sanity check:
    • 200 Watts / 17.5 volts Vmp = 11.4 Amps Vmp
    Using a simple voltage drop calculator for 28 feet of 10 AWG wire (one way run):
    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=17.5&phase=dc&noofconductor=1&distance=28&distanceunit=feet&amperes=11.4&x=89&y=33

    Result

    Voltage drop: 0.64
    Voltage drop percentage: 3.64%
    Voltage at the end: 16.86

    That is about the maximum voltage drop I would suggest...

    Another answer (more panels, more Watts, longer runs) would be to get an MPPT type charge controller and setup several panels in series (or series parallel, depending on panels and controller)--You can easily go 100' or more this way.

    -Bill

    PS: I should also add that MC4 connectors do not like to be connected/disconnected under load--The arcing is hard on the contacts too.

    Placing panels facing to ground and/or a PV Array disconnect switch (or using an AC plug) are ways to reduce/prevent damage to MC4 connectors.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,046 ✭✭✭✭✭
    edited July 2019 #3
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    MC-4 connectors will be damaged from the arcing produced when disconnected under load. Look into a set of Anderson connectors.   Also, don't disconnect the battery with solar still connected.

    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,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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