PV solar on RV -- some questions

billybardbillybard Registered Users Posts: 7
Howdy:

Just found this forum, and hope that it will answer many questions.

I'm planning on adding PV solar to my RV.
Thoughts are for two Kyocera KD-135SX panels, and a Solar Boost 2000E MPPT charge controller.
I have two Group 27 batteries (thinking about adding a third, maybe). I have a Heart 1500W MSW
inverter that I also want to eventually being on-line.

I'm concerned about several different aspects.
Hopefully, you experts can clear up my concerns (and, yes, maybe I am being far too nit-picky).

I've used some on-line V drop calculators to play around. Apx 7A and 4 feet between panels, and
either 14 or 21A and 24 feet from the panels to the controller.

From the voltage drop calcs, it seems like I should be using #8 wire between the panels,
and then #6 wires down to the controller (about 25 feet or so from the panels on the roof).

I'm concerned that most of the newer panels come with only #12 MC4 cables and connectors (and not just
a screw terminal J-box--like the now hard-to-find KD135SX panels).

I'm also concerned that the largest MC4 cables I can find are only #10 -- and will have too much
voltage drop if I ever add a third panel and start feeding 21A down to the controller.

To me, a 2% voltage drop is far too much in my situation, but please holler if I am being overly picky!
That's why I am posting on this forum -- you, the experts have far more experience from which I can hopefully learn.

Another concern that I have is when I am plugged into shore power at a camp site (where both my
converter and the charge controller will be trying to charge the house batteries). I'm not sure about
two difference sources each trying to charge the battery bank. (I can't think of a problem, but
I'd rather be sure about what is going on.)

OK, enough drivel for my first post. Lemme have it guys.

Cheers,
s/Mike

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: PV solar on RV -- some questions

    Mike,

    Welcome to the forum... To give you an idea--Here is a Video this nice thread from Kevin in Calgary Canada that shows designing and installing solar PV in a small RV trailer. Kevin did post a few weeks ago that the system is still working very well for him.

    The Kyocera panels are nice. I would suggest that you look at these controllers too. From smallest to largest (practical for your RV):

    Morningstar SunSaver 15 Amp MPPT Solar Charge Controller
    www.roguepowertech.com/products.htm 30 amp 12/24 volt MPPT controller
    Morningstar TriStar 45 amp MPPT solar charge controller

    One thing you can do with MPPT type charge controllers (those that have the ability to go with higher Voc voltages), is you can place your two panels in series. This allows you to have longer wire runs with less voltage drop on smaller AWG wiring.

    For MC4 connection panels, normally it is recommended that you don't cut the ends from the panels--that could make warranty service a bit of a pain.

    Instead, purchase a couple MC4 Male/Female jumper cables--Cut them in half. Then wire the cut ends into the rest of your wiring harness.

    8 Foot MC4 Extender Cable Male/Female
    Disconnect Tool for MC4 Connectors (set of 2)


    Normally, we recommend 1-3% maximum voltage drop for solar--trying to get below 1% becomes "expensive" for copper wire. If you can use an MPPT charge controller and connect the two Kyocera panels in series--That will drop your voltage drop way down.

    Regarding your inverter--1,500 watts is a big inverter for a small RV. If you are going to use the inverter a lot (such as charging laptop computer, charging cell phones, and running other small 120 VAC appliances), I would highly suggest looking at the MorningStar 300 Watt TSW 12 VDC inverter (600 watt for 10 minute surge). The problem with large inverters are; a) lots of wasted power just "turning on", b) MSW wave forms are hard on many small electronics, c) the Morning Star is one of the few 12 VDC inverters that can be programmed to enter "search mode" if there are no AC loads current plugged in.

    Also, a couple/three Group 27 deep cycle batteries are going to be hard pressed to supply 1,500 watt continuous and 3,000 watt surge current... For example, a few calculations:
    • 1,500 watts * 1/0.85 inverter eff * 1/10.5 inverter cutoff * 1.25 NEC Breaker/Fuse/Wiring factor = 210 Amp branch circuit/fuse minimum
    A typical Group 27 12 volt battery with ~90 AH hour of storage--2 in parallel would (recommended) supply:
    • 180 AH * 12 volts * 1/8 max continuous discharge * 0.85 inverter eff = 230 Watts (roughly 8 hours until battery dead)
    • 180 AH * 12 volts * 0.40 max surge power * 0.85 inverter eff = 734 Watts (maximum recommended surge from batteries)
    So--You can always have the Heart (or other large inverter) wired up and something you can turn on (running power tools, etc.)... But I would humbly suggest that you dial way back on the power requirements from that battery bank.

    By the way, in Kevin's RV thread, he installed a Battery Monitor--I highly suggest one of those if you are going to be serious about managing your battery power usage (and backup generator--if you plan on having one).

    Also, if you are going to be working with batteries and DC power a lot--I would suggest a DC Current Clamp type meter (cheap one here).

    And if you have flooded cells, get a good quality Hydrometer too.

    You are getting "Deep Cycle" batteries and not getting "Marine/Heavy Duty" batteries?

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: PV solar on RV -- some questions

    welcome to the forum,
    i'm a little confused why you would put #8 between the pvs as they have pigtail wires on them to begin with as one is about 33 inches and the other is about 39 inches. if the pvs are to be side by side then the pvs can be paralleled and the mc connections can be combined using these combiners,
    http://www.solar-electric.com/mumc4colam.html
    from there the use of a small #10 mc4 extension cable cut in 2 can be used and thicker wire then attached to the extension wire if you think you need better than the #10 extension wires.

    if the pvs are separated then this takes on a new flavor for paralleling and much more thick wiring to combine then with. in this case seeing as how you want an mppt controller anyway then you can go with a series arrangement on the pvs as the higher voltage allows a smaller wire to be used. if memory serves the solar boost 2000e does not allow for higher voltages so you may wish to consider one of the other ones bb pointed out if this is the case. the solar boosts are old mppt technology anyway. once a layout is made and please go into detailed description of the planned layout then i can comment on the figures you are wanting for low v drop %s. there's too many possibilities right now for me to comment on every possible layout with various wire gauges.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: PV solar on RV -- some questions

    If you used one of the Morningstar, Outback Power, or Midnite Solar MPPT charge controllers you can just put the panels in series (The solarboost cannot handle the higher voltage of doing that).
  • billybardbillybard Registered Users Posts: 7
    Re: PV solar on RV -- some questions

    Wow!
    Thanks guys, for your comments.

    I was planning on using the two solar panels in parallel to get a decent amount of charge current.

    I realize that most of the newer panels come standard with the MC4 connectors and cables, but #10 cables seemed a bit wimpy for 30A (even tho the NEC says that is correct, size wise). With the MPPT controller, maybe I shouldn't worry so much about the voltage drop!

    Currently, (no pun intended), I have a 600W MSW inverter already connected, and it is OK for "most" of the things that we do.
    But, of course the little lady would like to have the coffee pot on the inverter as well. The 600 W inverter already handles the sat receiver, the TV, surround sound, and three computers without any problem. (The larger inverter is just a "want to install" at this point).

    My real concern was trying to cram 14 or 21A in a #10 MC4 cable, down some 25 feet from the roof to the charge controller. Guess I am being
    overly picky about worrying about the voltage drop????

    Thanks for your responses, and keep 'em coming if more thoughts crop up.

    Cheers.
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: PV solar on RV -- some questions

    Say you want 3x 135 watt Kyocera panels... First, that is a relatively high voltage (Voc would probably exceed the smaller MPPT charge controller's maximum input voltage. A larger controller, like the Morning Star 45 amp TS MPPT type charge controller can operate upwards of 140 VDC--But you are looking at a ~$400 charge controller...

    But, if you want 1% voltage drop, 17.5 Vmp / 7.7 amps * 3 panels in series and 10 awg wire, using a generic voltage drop calculator:
    • 52.5 volts @ 7.7 amps -> 25 feet (one way run) and 0.46 volt drop

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
    Re: PV solar on RV -- some questions

    Ours is also a RV situation. The question I would ask is why not start out with higher voltage panels to begin with? Before I had done my homework I made the mistake of buying a panel intended for a grid tie system with OCV over 50V which meant that I had to buy a Morningstar MPPT controller. I also bought recently a Unisolar flexible panel that puts out 44V OCV for which I found a STECA PWM controller good to 47V.
    I knid of wonder what affect having high voltage panels will have on shading losses. I get to find out this summer when we head for California and camping in a couple of National Parks with no power.
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: PV solar on RV -- some questions

    Shading--I don't know. The best answer is no shading.

    If you have three "12 volt panels" in series, and shade one of them, and MPPT controller will see about 1/3 less voltage and output 1/3 less current into the battery bank while the panel is shaded.

    If you shade 2 of 3 panels, the additional voltage drop of two shaded panels may cause Vmp-array to be too low to charge a 12 volt battery bank (bypass diodes have 1-2 volts of drop per "dark" panel).

    Would it be better to install panels in parallel... Not sure, if you block the sun partially on one "12 volt" panel, it will not generate any useful power. Higher Voltage panels, you may get some power--but it depends on how the bypass diodes are installed and what shading pattern is hitting the array (i.e., covering only one string of cells vs partially blocking two or more strings of cells).

    One person here who gets lots of snow laid the panels in "landscape" so that the snow on the bottom of the frame would leave the upper string fully exposed.

    If you have vertical vent pipe, chiminey, power lines, trees, etc. partially shading an array--Best bet is to move the pipe, move the array, or trim the trees. Or, at least, keep the shading from ~9am to 3pm to the smallest amount possible.

    Some folks with (for example) East shading will mount the array facing South West and still gather a reasonable amount of energy.

    You can play with the numbers using PV Watts (panel orientations).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: PV solar on RV -- some questions

    you can still parallel pvs as with one shaded you will still get the power generated by the other pv. what's best is a hard call to make because it depends on your circumstances you believe you will encounter for shading issues. shading is highly variable enough for a fixed location let alone a moveable one. i think you might come out better in most cases with a series arrangement when using 12v nominal pvs.

    now i understand it when you said the pvs are to be paralleled electrically, but i wanted to know their physical proximity to each other.

    if you are going to use an mppt controller that can downconvert then you could opt for higher voltage pvs. instead of getting a vmp in the 17.5v-18.5v range (nominal 12v pv nowadays) you could get pvs with vmp ratings like 29v or 36v as examples and would depend on the controller you choose as many smaller ones have limitations on their input voltage. the voc is something to watch out for too as pv voltages with a full battery can go to the open circuit voltage and exceed the max voltage to the controller. cold pvs will also have slightly higher voc ratings.

    it wasn't my intention to confuse you, but you have to understand you can do this in other ways too even though the ifs, ands, and buts you may not understand right away from how i explained it. you could also have temporary ground mounts if you don't intend to stray from the rv for them to be stolen for then you can park the rv in the shade.
  • billybardbillybard Registered Users Posts: 7
    Re: PV solar on RV -- some questions

    Hi again:
    Thanks for the additional replies. All good info.
    Guess I misled the group by nit-picking voltage drop so much.

    Since I have limited real estate for placing solar panel, I was planning on putting either two or three solar panels in parallel.
    In my situation, I believe that I need as much current as I can get from the panels.

    When I am boondocking, I am not worried about shading, since that is usually not a problem when I am out and about.

    I realize that by using a series configuration that I can get higher voltages and thereby less voltage drop, but then I am severely limited in the current that I need to recharge my limited battery bank.

    Thanks for the leads to other threads and some neat info on MPPT charge controllers.

    Keep the thoughts and comments coming ...
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: PV solar on RV -- some questions
    billybard wrote: »
    Hi again:
    Thanks for the additional replies. All good info.
    Guess I misled the group by nit-picking voltage drop so much.

    Since I have limited real estate for placing solar panel, I was planning on putting either two or three solar panels in parallel.
    In my situation, I believe that I need as much current as I can get from the panels.

    When I am boondocking, I am not worried about shading, since that is usually not a problem when I am out and about.

    I realize that by using a series configuration that I can get higher voltages and thereby less voltage drop, but then I am severely limited in the current that I need to recharge my limited battery bank.

    Thanks for the leads to other threads and some neat info on MPPT charge controllers.

    Keep the thoughts and comments coming ...

    You may have missed some of the explanation here. By putting the panels in series you increase the Vmp of the array. This means you must use an MPPT type charge controller. With one of those the output current will not be limited by the current of the array. So if you have three Kyocera 135's in series you get:
    Vmp 17.7 * 3 = 53.1, Voc 22.1 * 3 = 66.3, Imp 7.63, Isc 8.37
    For the array. As far as potential charging current coming from the controller goes it is more like: 135 Watts * 3 = 405 * 77% efficiency = 312 Watts / 12 Volts minimum battery Voltage = 26 Amps.
    As you can see, in some circumstances that may be more than the straightforward panel Imp * 3 (22.89 Amps). It certainly would not be less.
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: PV solar on RV -- some questions
    billybard wrote: »
    I realize that by using a series configuration that I can get higher voltages and thereby less voltage drop, but then I am severely limited in the current that I need to recharge my limited battery bank.
    I don't realize that at all. If you are using an MPPT controller, the controller will maximize the power. It does this by dropping the voltage while increasing the current. The advantage of MPPT is that you transmit the power from panels to controller at high voltage (less power loss in the wires) and transmit the power from controller to battery at lower voltage (higher current).

    MPPT has some advantages in maximizing power (especially in cold conditions), but many people use it primarily because they have long runs between panels and controller and want to minimize the amount of copper (money) between panels and controller.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: PV solar on RV -- some questions

    For MPPT (ignoring losses):

    • Power = Voltage * Current

    Example:

    • Power = 3x 17.5 Vmp * 7.7 amps per panel = 404 watts = 14.5 volts Vbatt-charge * 27.9 Amps Ibatt-charge

    More or less, a MPPT charge controller is like a DC version of a variable AC transformer. The power in = power out; And the MPPT controller optimizes the Vmp-array*Imp-array to get maximum power from the array (when maximum charging current is needed by the battery) and then efficiently "down converts" the high voltage/low current from the array to low voltage/high current needed by the battery bank.

    Note--We do use a derating factor of 0.77 (dirt, wire loss, heat, a bit of panel aging) to get a more realistic day to day maximum current rating:

    • 27.9 amps "marketing rating" * 0.77 = 21.5 amps "typical" into 14.5 volt charging

    Also note that because this active power tracking (maximum power point tracking MPPT), the output current is also dependent on the battery bus voltage... So if the battery has been discharged and is recharging at 13.5 volts instead--you will get a bit more current during that time.

    The major downsides to MPPT controllers---They are not cheap. And they are not as efficient for smaller arrays (smaller than ~200-400 watts, a PWM controller may be more efficient because it uses less power than the MPPT "switching power supply" system). But--it depends on length of power run and other issues too (Vmp vs Vbatt should be roughly 17.5-18.5 Vmp to 12 volt battery bus for a PWM controller to be "efficient").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
    Re: PV solar on RV -- some questions

    I understand your situation and can guarantee I have even less room for panels with a 6X10' roof. I have only one 185W panel mounted permanently about all I have room for. What a number of us that have very limited space have done is to have portable panels that can be put out in the sun. Mine is flexible panel a friend has two panels hinged together.
    One the most important components is conservation, and this is particularly important with limited solar and limited battery imposed by an RV. When I had our teardrop built I learned from our first one and I provided the LED lights and specified that i did not want a Fantastic fan installed but did want the wiring. I used instead a couple of three speed computer case fans at a significant power savings. I have an Eberspacher diesel heater with a significant fuel cost savings as well as a current savings over a LP RV furnace. The TV draws 20 watts and the sat receiver draws 23.
  • billybardbillybard Registered Users Posts: 7
    Re: PV solar on RV -- some questions

    Aha!

    Thanks guys. The light is slowly illuminating in my feeble brain.
    I am now slowly recognizing the benefits of an MPPT controller if I use a series configuration instead of a parallel configuration.

    Thanks to all you experienced experts to help get my brain on the right track. I still have a lot to learn, but I feel that I have learned a bunch already.

    Back to the drawing board to think through some more scenarios...

    Cheers.
  • billybardbillybard Registered Users Posts: 7
    Re: PV solar on RV -- some questions

    The light seems to be glowing more brightly...

    Just spent some time playing with calcs for parallel and series panel arrangements.

    Appears that the series arrangement wins out, even with only two panels (if I ignore the extra cost of the MPPT controller).

    Makes it a lot easier to expand later on if the need arises.

    Thanks for agitating the marbles in my feeble old brain. Maybe there is hope yet!

    Again, the many opinions shared by so many of you have been a real help in getting me more acquainted with some of the less obvious ramifications of solar.
    Thanks.
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: PV solar on RV -- some questions

    Pretty much, stepping through one or two paper designs--And you will become one of the experts here too.

    Once you have gone through the design by rules of thumbs--Then the details on why the rules were created in the first place--Start to make a lot more sense.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: PV solar on RV -- some questions
    BB. wrote: »
    Once you have gone through the design by rules of thumbs--Then the details on why the rules were created in the first place--Start to make a lot more sense.

    -Bill

    I thought they were created because we're all too lazy to do all the calculations with every system loss percentage at every step of the way over and over and over. :p
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: PV solar on RV -- some questions
    I thought they were created because we're all too lazy to do all the calculations with every system loss percentage at every step of the way over and over and over. :p

    Naaa... It is just because I am all thumbs. :p:p

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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