RV Solar wiring help needed

rexxrexx Registered Users Posts: 6
I have read tons, and am so freakin confused right now. I am going to be putting solar on my rv. I can affordably wire them in parallel or in series.

I am thinking about running 3 larger higher voltage panels to a mppt converter. I have read many times that series is a bad idea for an rv because if one panel goes down do to shading they all go down. Yet I know that most larger panels have 3 bypass diodes in them. So does it make any difference how I wire them?

Are there any panels with more then 3 diodes for better shadow tolerance besides unisolar?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: RV Solar wiring help needed

    Welcome to the forum Rexx!

    There are several ways of defining your system... One is to measure/specify the loads, then size the battery bank and solar array.

    Another is to use the battery bank that fits, and then how much solar power to keep the batteries happy.

    And, you have XYZ watts of solar panels, what can it support...

    At this point, it is not clear what the best answer will be for you.

    However, two points against Unisolar--Point one is they are out of business--So you may find panels at a good price for now, or not. Second, Unisolar panels are "thin film" type panels. While thin film panels can have advantages (possibly less easily damaged by rocks/etc.), the are, roughly, 1/2 as efficient at collection solar energy.

    So, if your roof space is small, then standard crystalline solar panels will collect about 2x the amount of energy per sq.ft. of roof area. If you need to collect as much power as you can from your roof top, then you may want to look at other panels than Unisolar.

    Connecting panels in series vs parallel--Lots of electrons have been sent over the web about this issue--And I am not sure that there is a "good answer" that will come out of all of that discussions. One of the reason is that shadows travel over the panels during the day--So what may be true at 10am shade may not be true at 2pm shade--Other than you should avoid shading between ~9am to 3pm for maximum power collection.

    And depending on where you will be camping, having panels that can tilt may help too (the farther north you go, and the more winter camping you do, the more tilting of panels will help collect more power, especially during late fall/winter/early spring.

    The choice between MPPT and PWM charge controllers is a bit more complex. In general, smaller systems (perhaps upwards of 400 watts or so), tend to do fine with less expensive PWM controllers. Systems over 800 watts usually work out better with MPPT type controllers.

    MPPT are good choices if a) you have a long wire run from array to charge controller/battery box; b) if you have solar panels that do not have Vmp~17.5 to 18.6 volts range (MPPT controllers can efficiently down convert high voltage/low current arrays to low voltage/high current for battery bank); or c) you purchase larger solar panels (over 100 watts or so)--Larger solar panels tend to have Vmp>>17.5 volts and they are cheaper on a $$$/watt basis--So you can save on solar panels but will pay some back on more expensive MPPT type charge controller to run your system.

    Also note that different MPPT controllers have different maximum input voltage requirements... Some are as low as 40 VDC, others 70 VDC, and other 140-150 VDC or more (in general, the higher the input voltage, the larger the charge controller and the more expensive). So, there are more variables to juggle when sizing/configuring your solar panel array.

    Sorry for all the hand waving, but if we can know a bit more about your system requirements (AH/WH per day, roughly when and where you camp, any equipment/limitations you may have such as AH for battery bank, you can fit only 2x140 watt panels, etc.).

    Depending on your needs and size of the system, the answers can change a lot.

    Here is a nice thread with video from Kevin in Calgary Canada that shows designing and installing solar PV in a small RV trailer. Perhaps it can give you some ideas too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rexxrexx Registered Users Posts: 6
    Re: RV Solar wiring help needed

    Here is my theoretical setup
    three or four 200w panels that have the standard 3 bypass diodes
    one 45amp MPPT controller
    one 12v battery bank

    The cost of wiring them parallel vs series is negligible I can do both easily.
    Shadows are going to be the norm on a rv rooftop. I am bound to park near signs and light poles and trees buildings etc.

    My question is...Is there any performance difference in wiring them in series or parallel? Can you point me to a thread on this exact topic?
    Does parallel wiring trump bypass diodes?
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: RV Solar wiring help needed

    in most cases of various shading events the series connection will fair better. this is because when you have paralleled pvs and say the bottom 50% of all of the pvs get shaded you won't have any output as all of the pvs are below the minimum voltage to properly charge the battery. that same shading event with series will output useable power. if shading will be the norm for you i would suggest opting for some remote pvs you can put into the full sun while the rv is in the shade or partial shade. you can do both the remote and rv mounted pvs, but i would say you will need another controller to handle the remote pvs as their voltage will differ greatly at times from the voltage of the rv pvs.
  • rexxrexx Registered Users Posts: 6
    Re: RV Solar wiring help needed
    niel wrote: »
    in most cases of various shading events the series connection will fair better. this is because when you have paralleled pvs and say the bottom 50% of all of the pvs get shaded you won't have any output as all of the pvs are below the minimum voltage to properly charge the battery. that same shading event with series will output useable power.

    I understand how this is true with 12v panels. Is this still true with high voltage panels being down converted to 12v?
    Am I correct that panel volts will only drop like 2v because of shading before they stop producing completely?


    Thanks for all the fast replys.
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: RV Solar wiring help needed

    There is no good way of managing shade and having lots of output current from a solar array:

    http://www.windsun.com/General/tech_tips.htm
    All panels, regardless of what the advertising says, will lose considerable output even if only partly shaded. Some, such as the Unisolar panels, lose less in partial shade, but the output is still reduced. In crystalline panels, when a cell is shaded, it essentially "turns off" that cell, turning it into a high resistance. If a single cell in a panel is shaded, it can reduce or even completely cut off the output of the panel. In some cases, it can also result in overheating of the cell as the unshaded cells try to force current through the high resistance cell. Unisolar panels are less susceptible due to built-in bypass diodes on the cells, but will still lose up to most or all of their power with 15-20% shading due to the voltage dropping below the battery voltage.

    Your best bet--Move the panels or remove the shading to the best of your ability (cut trees, move vent pipes, park RV with shading away from sun, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: RV Solar wiring help needed

    as far as high voltage pvs go as long as there is enough voltage being produced by the unshaded portion of the pv then it will produce some power. shading does take away from the potential power a pv would normally produce in every case. voltage wise there isn't really any difference between 2 12v pvs with 36 cells each in series with one another than a single pv with 72 cells for 24v. if half of the 24v pv is shaded it will be 12v minus losses through the bypass diodes and will still be high enough to send a charge to the battery. shading more on that 24v arrangement will kill the power.
  • rexxrexx Registered Users Posts: 6
    Re: RV Solar wiring help needed

    So the downside to series wiring is that there is a .3v drop for every bypass diode that is triggered?
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: RV Solar wiring help needed

    You do not need a blocking diode for systems with Charge Controllers--The controllers will prevent current backflow through the array at night (and discharging the battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: RV Solar wiring help needed

    yes, there will be a v drop for each bypass diode the power goes through and .3v may or may not be the v drop for your pvs as i'd have thought it to be more closely to .5v/diode.
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