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Solar panels in parallel - Partial shading question

Discussion in 'Off Grid Living' started by melbo, Mar 8, 2013.

  1. melbo

    melbo Hunter Gatherer Administrator Founding Member

    I'm working on a solar project for my RV. If I install two (12 Volt) 140 Watt panels in parallel, run to a single charge controller and then to my battery bank, what happens if one of the panels gets partially or fully shaded during the day?

    I'm going with two smaller wattage panels due to space constraints on the roof and because of other stuff on the roof, there will be times of the day where I'll get some shadowing. If partial shade on one panel kills the output from both, would I be better to run two smaller charge controllers down to the battery bank? I've read that some (most?) panels now have blocking diodes built in.

    The battery bank is 4 (6 Volt) Trojan T105-REs wired for 12 Volts.
  2. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Nothing, for the other panels output, but the shaded Panels output will be degraded, due to the loss of illumination. Same thing as any current sources in Parallel....if they were in Series, it would cut the output to the lest illuminated panels current....
    melbo likes this.
  3. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Oh come on now. it can't be that simple... :love: I was hoping for an explanation along these lines:

    Tully Mars, ghrit and VisuTrac like this.
  4. BTPost

    BTPost Old Fart Snow Monkey Moderator

    P=IE. E = I/R. With those two you have ALL OF DC Power covered...
    VisuTrac and melbo like this.
  5. techsar

    techsar Monkey++


    But it really is that simple (this time.)
    VisuTrac likes this.
  6. melbo

    melbo Hunter Gatherer Administrator Founding Member

  7. BTPost

    BTPost Old Fart Snow Monkey Moderator

    If your asking what I would get.... I would spend my money on an Outback MX60 or the newer version of that... Before you scream it is way to expensive... You asked... And they are the BEST MPPT Charge Controller on the market, PERIOD, Hands down... And Andy will tell you the same thing...
  8. techsar

    techsar Monkey++

    and to get the most out of the MX60, the MATE controller...allows you to set charging parameters to your preferences. As BTPost said, they are expensive (compared to many others) but they are top of the line gear.
  9. melbo

    melbo Hunter Gatherer Administrator Founding Member

    I don't typically look at cost when I'm looking to purchase something durable and gravitate towards the best - much to the chagrin of my wife ;0. I had a feeling that an MX60 or Flexmax would be overkill for the ~16 amp system. My roof space is limited so I cannot expand the system.

    Would a high end outback 60 amp controller outperform a high end lower amp unit?

    Sorry for the elementary questions...
  10. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Yes, it will and can be used post SHTF to control a much bigger system and it would allow you to change both the Battery Bank Voltage, as well as the Solar Panel Buss Voltage to meet any desired changes in the system...
  11. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Now that makes sense. By that rationale, I should go with the MX80...
  12. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Always size for max. Burnout is possible/probable unless you have a current limiter circuit in line. Home built limiters are possible using Zener diodes, but that isn't for novices.
  13. BTPost

    BTPost Old Fart Snow Monkey Moderator

    The price difference is only about 20% so if that doesn't scare you off, that is what I would do... Also just because you only have so much sq ft on the flat top doesn't limit what you could use, once you get parked for a while...
  14. kellory

    kellory An unemployed Jester, is nobody's fool. Banned

    I love having this kind of info on tap like this. It is a resource. :cool:
    Tully Mars and Quigley_Sharps like this.
  15. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Been thinking about some other uses for this system (other than while dry camping).

    When the travel trailer sits in the yard and is not being used, perhaps I could run the AC out of the Inverter to a subpanel with transfer switch next to my main AC panel. That would allow my system to provide some of my household needs while it's sitting idle.

    So I've found a way add another 140 Watt panel to the roof of the RV for a total of 420 watts at 12 V.
    Run in parallel to an Outback MX80 MPPT Charge controller.
    Charging a 440 Ah battery bank (6 Trojan T105 wired for 12 V)
    Hooked to a Magnum MS series 2000 Watt inverter.

    When I wanted to help power some circuits in the house, I'd connect the inverter to the new subpanel and use a transfer switch to transfer those circuits from the main AC panel to the subpanel, powered by the inverter.

    Now I know that this is the backwards way of trying to calculate things but keep in mind that its a bonus use of my Solar Generator on wheels... :)
    How do I caclulate what and how many circuits to add to the subpanel?
    Or, what's the maximum amount of draw from my battery bank to keep things in a comfortable working capacity of the battery bank?
    I suppose we need to know how well the 420 Watts of panels can be expected to charge the 440 Ah bank.

    Does this make sense?
  16. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Start, of course, with a list of those loads and see if you can even begin to power them from the camper. Consider that for best battery life the rate of draw will be the controlling feature, you do NOT want to suck it dry. Add up the amps draw for those loads, and see how that matches up with the battery capacity. If you have a motor in mind, be cautious to include starting current in your capacity calcs.
  17. melbo

    melbo Hunter Gatherer Administrator Founding Member

    I can do that and didn't intend to run any motors, mainly circuits that power LED lighting, parasitic loads like laptop docks and other small stuff. Would there be a way to arrive at a number of draw that would be safe for the draw off the bank and would easily be replenished by the panels? 300 watts? 500 watts?

    If I had a number, I could work backwards with a kill a watt to figure out what I could add to the subapanel.
  18. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    The draw could be very high, limited by battery cooling and conductor size (but not for very long --. Often forgotten is internal resistance in batteries, that's what overheats them in discharge and charge if the current is too high.) It isn't the wattage draw that will control things, but the ampere hours in the cans. Almost the same thing, but not quite.

    As a practical matter, there should be some info around on the batteries themselves as to a designed sustained draw. The draw has nothing to do with rate of recharge from the panels, but you could do a SWAG balancing the ins and outs knowing the number of hours recharge would take (both with and without any loads) and back figure the amp hours you could draw (say) over night. The batteries will also have owner's manual type data indicating the max drawdown, probably as a percent of full ah capacity. At the very least, you should have a capacity curve showing ah vs. voltage during loading. You really do not want to drop the voltage below 12 volts or in a desperate situation, 11.5 volts. (Proportion that for 24 or 48 volts, same difference assuming lead acid. Other types will differ.) Go too far low, and one or more cells can reverse, end of life for the batteries no matter how new. We used undervoltage relays to protect the batteries, they would shed the load at some predetermined output voltage, loaded or not. There will be some recovery after a load shed, but nothing to hang your hat on. (Example, car no start, run the battery down, wait a while and try again.)

    Perhaps a side issue, perhaps not. There are several charging schemes in use, knowing the one your mfr recommends is a pretty good thing to know. I have a bit of background with vented lead acid cans, but the newer flavors are mysterious. The new ones are different, so are the charging schemes.

    Yes, you could recharge the cans from house power, but that makes no sense at all. Way too inefficient to shove electrons around the horn, losses add up on each conversion.

    Also, check the desired voltage on the loads you intend to power. Most of the newer electronics are most efficient at 13.8 volts. I wish I knew why.
  19. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Thanks for the detailed info.

    Attached are the spec sheets for:

    • Trojan T-105 RE Battery
    • Outback FlexMax 80
    • Magnum Energy MS Series inverter
    • Solarworld Sunmodule 140 Watt panel.

    Attached Files:

  20. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Ok M. You need to understand that the Current Draw from the Battery Bank needs to be LESS than 100Amps, so that means if you plan for more than 1.2 Kw of load, you should bump the Battery Buss Voltage up to 24 Vdc. Also understand that Bigger Batteries, are much better than multiple smaller Batteries in series/parallel. You are better off with L16HDs than T105s. This is with the understanding that you have room for the L16s, as they are taller than the T105s. None of his has any impact on the Panel Buss Voltage, as the MX80 will deal with the conversion. Another thing to remember is that should you have extra room on the Roof of the Barn, on the Southern exposure, you can mount more Solar there and then just add that into the Input to your MX80, while it is Parked, and disconnect those when you take the Land Yacht, for a cruise. The only real design criteria that needs to be decided NOW, is the Input Voltage for the Inverter. Rule of Thumb is never more than 100 Amps from the Battery Bank. So for your 2 Kw Inverter, you should be thinking 24Vdc... .....
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