Noob - 2 panel fixed/portable RV build

Dalama32
Dalama32 Registered Users Posts: 4
in Solar Beginners Corner #1
I'm planning on installing a two panel system with my RV. One panel will be installed permenantly on the roof and the other will be a portable one that can be moved around or perhaps not used at all. The panels are identical 95w 17.8V. The portable panel will have a run of 30 feet and the permenant one 10 feet. Inevitably these two panels will not have the same sun exposure.
Do I need seperate controllers or can I run each panel to a single controller (parallel connection)? When calculating voltage drop and the 3% rule, I assume that it is additive; panel1-controller + panel2-controller + controller-battery < 3%? Will I need a blocking diode? Any other considerations? Thanx.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Noob - 2 panel fixed/portable RV build

    Welcome to the forum.

    There should be no trouble running these two panels through the same PWM type charge controller. The Imp on them is about 5.4 Amps, so together they would be 10.8 Amps at most. You should therefor use a charge controller of >10 Amps capacity like Morningstar's SunSaver 20 or ProStar 15 http://www.solar-electric.com/stpwmchco.html

    The V-drop calculations apply to particular wire runs. The biggest Voltage drop concern is on the 30' run from the "portable" panel. A quick calculation shows that 10 AWG wire should handle this with minimal loss; less than 3%. No blocking diode is required: the charge controller will prevent the battery from discharging through the panels at night.

    This is a popular configuration for RV's: fixed panels & movable ones to get the sun when you're not in it. The other consideration is the vulnerability of the portable panels; they are more likely to get damaged or stolen and that is something you have to consider. The wires can be a bit of a hazard too, as it is possible to trip over them. They won't shock anyone who does, but they are easily broken. Some very heavy outdoor extension cord type wire can be your friend here. Then you only have to worry about it getting snagged and pulling the panel over. Just things to think about and plan for.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Noob - 2 panel fixed/portable RV build

    this isn't critical as you state it so the 2 can be run into the same cc even though the exposure can be mismatched. if one of the pvs should get shaded a small amount of current could back feed into the shaded pv and 2 ccs would solve that, but at a cost. blocking diodes would be a cheap way to isolate the pvs from any back feeding of power, but when the pvs are illuminated the diode sets up a loss of about .5v to .75v and when you multiply that by the current that is the power loss seen all of the time so this too may not be worthwhile doing. if you ever expand to say 3 or more pvs or you replace one of the 2 with a higher power pv then it is possible you will need the blocking diodes to prevent a pv from being fed too much back feed current and damage the pv(s).
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  • Dalama32
    Dalama32 Registered Users Posts: 4
    #4
    Re: Noob - 2 panel fixed/portable RV build

    Thanks Guys,
    One other question... for this setup would it be better to go with slightly higher panel voltage? I have choice on the 95w panels of 17.8v/5.34a or 18.34v/5.18a
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Noob - 2 panel fixed/portable RV build

    Not really. You already have enough Voltage & wire gauge to minimize the resistance over the run. Adding half a Volt won't change much, and if you're using a PWM type controller the extra won't add any more power as they are current-dependent.

    Technically you'd actually lose a tiny bit of power, as 95 Watts / 17.8 Volts is 5.33 Amps whereas 95 Watts / 18.34 Volts is 'only' 5.18 Amps. In practical terms you'll probably never see either maximum current.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Noob - 2 panel fixed/portable RV build

    for a pwm cc go with the lower voltage pv.

    btw, i forgot about your question of the v drop. running parallel pvs means they are independent circuits at the point of division. you would figure it out with all common aspects and then one of the separate pv circuits and then do the same for the other pv circuit. you do not add the losses of pv1's circuit with pv2's circuit when in parallel. if they were in series then you would be correct to add them together as then they are all part of the same circuit.
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