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  • Off grid system for weekend use

    Hi all,
    I own a camp in Maine (Rumford Maine area, specifically). I've been doing research on how to properly upgrade the current solar panel system.
    So first off, thanks for hosting such a wealth of information here. It's been great.

    A run down of the current system:
    • 1 x 110W 12v panel (from 2001, so it's probably lost a good deal of efficiency)
    • 1 x Solar boost 2000e MPPT charge controller
    • 1 x 2000w Inverter (12v in only)
    • 6 x Energizer gc2 batteries wired for 12v (dated 2011)
    Currently, after a work week worth of charging, the battery system will handle a full day / night's worth of load. It does get close, however.

    Essentially, I think the panel is vastly under-powered for my needs.

    Purposed changes:
    • 3 SolarWorld SW290 Panels. Probably wired in parallel to help deal with any potential shading issues.
    • 1 x Tristar MPPT Charge controller (http://www.morningstarcorp.com/products/tristar-mppt/). Probably 45amp
      • I know this leaves some panel capacity on the table at 12v, but that will be solved by moving to 24v or higher when batteries are replaced
    • Keep the batteries and inverter for now. Batteries are probably nearing their useful life.
    Down the road:
    • Battery upgrade
      • At least 24v to utilize panel capacity better
      • This means ~ 320Ah of capacity?
      • ~80 Ah 6v batteries x 4 in series
        • Probably will go higher than this for fudge factor
        • Ideally, staying under 8 batteries total
    • New inverter
      • Just needs to be able to handle a more flexible input (24v probably)
    • Could add more panel capacity if needed
      • Maybe my load ends up being more than expected
      • New loads
    Estimated loads:
    Name Watts Hours Watt-Hour Notes
    TV 30 5 150
    Laptop 75 4 300 Charged during the day.
    LED lights 8 6 42
    CFL lights 15 3 45
    Phone charger 24 5 120 Charge during the day
    Coffee Maker 800w .25 200 Have a percolator if the load ends up being too high.
    fudge factor 100 2 200
    Total 1057Wh
    Some notes -
    • I know the batteries are undersized for this setup
      • They're 5 years old, I want to extract the remaining useful life out of them before upgrading
      • I believe the current setup has been chronically undercharging these batteries, though it has typically all week to reach a full charge
    • Load estimation
      • I try to combo up all charge related activity for when the panels are putting out current
      • Odds are some of these loads are higher than they actually are. Nice to be on the safe side
      • Some load drops off in the winter
    • Usage
      • Weekend use typically
      • Winter use is less frequent, but occurs
    • I've got a small gas generator for battery top ups and crappy weather
    Lastly, a few questions:
    Am I correct in thinking that wiring my panels in parallel helps shield me from potential shading issues?
    Should I go to four panels instead of 3?
    Am I making any huge mistakes?

    Thanks!
    Last edited by beatus; 06-22-2016, 09:08 PM.

  • #2
    a 24 volt system is better, L-16 batteries would be even better, and a 48 volt battery bank and 48 volt inverter using l-16 would more than cover all of your fudge factors. I am surprised parallel batteries inadequately charged by solar lasted 5years. your lucky they should have died long ago.
    4X Suniva 250 watt, 8X t-105, OB Fx80, dc4812vrf

    Comment


    • #3
      Originally posted by Logan005 View Post
      a 24 volt system is better, L-16 batteries would be even better, and a 48 volt battery bank and 48 volt inverter using l-16 would more than cover all of your fudge factors. I am surprised parallel batteries inadequately charged by solar lasted 5years. your lucky they should have died long ago.
      Logan005,
      Thanks for the response! I just purchased the place in December, I think the previous owners only ran lights and some small stuff so the batteries stayed near topped off. Either way, I think the drop in capacity is noticeable on them from what I calculated they should be holding.

      Appreciate the recommendation on the batteries - I'm going to try and limp through this summer and winter with the existing then upgrade.

      Something I forgot to ask in the OP, Should I consider getting four panels instead of three? I've read a lot around these parts about the dangers of undersizing panels, leading to pre-mature battery death.

      Comment


      • Logan005
        Logan005 commented
        Editing a comment
        on a single string, odd number of panels is not a problem. you can not switch to 2 strings with 3 panels. so you would need a matching 4th panel. Your list of power needs is very lofty, when living off grid "Less is more" learn to use much less power at night, and you will save thousands of extra batteries.

      • beatus
        beatus commented
        Editing a comment
        Thanks Logan.
        I think 600 of the 1057 is day time use (Charging laptops / cellphones during the day, using battery at night, coffee maker will be morning, so borderline there).

        Dumping the fudge factor, I think it's reasonable that my nighttime usage will only be around 300Wh. Just trying to overbuild here to make a system that will last a long time and do minimal depth of discharge on the batteries.

    • #4
      I would replace the coffee maker with a stove top type and make coffee from wood stove or gas stove. a heating element is not a good load for expensive and easy to damage batteries. Supersonic makes a 13.3" LED TV that only uses 6.5 watts @ 12 volts @ 720p, I would replace CFL bulbs out to LED. Everything else you have listed can be powered directly from battery w/o any need for an inverter. For a small cabin, an inverterless system is more efficient and safer for your batteries.
      4X Suniva 250 watt, 8X t-105, OB Fx80, dc4812vrf

      Comment


      • #5
        I see two issues you need to address, and one fire danger.
        • You are way under powered in panel watts. A 110 watt panel is not even enough to maintain 2 of your dead batteries.
        • Your batteries are over due for replacement. Do not make the same mistake the previous owner made using parallel batteries.
        • A 2000 watt Inverter on 12 volts is DANGEROUS.
        Now let's pick apart your proposed changes, you have issues.

        So you want to use 3-290 watts of panels. That is a total of 870 watts. If you insist on staying with 12 volts which is a huge mistake. It will require a 80 Amp Charge Controller. 870 watts / 12 volts = 72.5 Amps. 45 amps will not cut it. If you move to 24 volts, you will save some serious coins because it only requires 870 watts / 24 volts = 36.25 amps. But there is a catch. Not sure what the Vmp of these 290 watt panels is, but if less than 36 volts (72 cells), you will have to wire them in series to run a 24 volt battery.

        Note on panels. 3 is a PRIME number. That means you only have two configuration options. All in parallel, or all on series. You can get away with prime numbers of 1 and 3. Even 3 can be a problem. If you have to wire the panels in series, the total Voc might exceed the limit of the controller. Having said that the MS Tristar 45 amp MPPT has a 150 Voc which should work with the 3-290 watt panels.

        Batteries. Replace them, they are over due. At 1 Kwh usage is not a lot, and 870 watts of panel power should be able to generate you 2 to 3 Kwh per day in Summer months. You have to be careful to match panels, controller, battery, and Inverter. Batteries have a minimum/maximum charge/discharge requirement. Go outside those limits and you will run into problems. You want to keep the current no lower than C12 on charge, and no greater than C/8 on charge and discharge. Where C = the battery Amp Hour Capacity. So example a 200 AH battery minimum requirement is 200 AH / 12 Hour Rate = 17 amps, and 200 / 8 = 25 amps. Golf cart batteries you can go up to C/5 or 40 amps in the example. That also limits Inverter size. Example a 200 AH golf cart battery at 12 volts is 12 volts x 25 amps = 300 watts. Golf Cart Battery = 12 volts x 40 amps = 480, call it 500 watts.

        You need to change course. Forget 12 volts to start. Based on 1 Kwh per day usage I am going to bump up to 1.5 Kwh per day for you and recommend:.
        1. Panel Wattage = 400 to 600 watts using 2 panels. Try to avoid Prime Numbers and wire in series. 2-250 watt panel would be perfect at 500 watts.
        2. 24 volt @ 200 AH. Use 4-6volt Golf Cart batteries wired in series.
        3. 15 to 25 Amp MPPT Controller depending on panel wattage. 400 watts = 15 amps, 500 watts = 20 amps, 600 watts = 25 amps. No harm in going higher for future expansion if the wallet can take the pain.
        Use the money I just saved you to buy a 24 volt 1000 watt Inverter. If you use high quality Golf Cart battery like Trojan T-105 you can use 2000 watt if needed based on maximum demand.

        Please do your homework and understand wire and cable sizes and how they relate to current, and the safety issues involved. A 24 volt 2000 watt Inverter requires 100 amp of current. One loose connection or undersized wire is a FIRE. 24 volts will not electrocute you, but 100 amps can make one nasty fire and explosion. So please take time to understand what you are asking for.

        Edit Note:

        Since this is a weekend system, what I proposed can easy do 2 Kwh day in Summer months and 1 Kwh/day in Winter.
        Last edited by Sunking; 06-23-2016, 10:45 AM.
        MSEE, PE

        Comment


        • #6
          Originally posted by Sunking View Post
          I see two issues you need to address, and one fire danger.
          • You are way under powered in panel watts. A 110 watt panel is not even enough to maintain 2 of your dead batteries.
          • Your batteries are over due for replacement. Do not make the same mistake the previous owner made using parallel batteries.
          • A 2000 watt Inverter on 12 volts is DANGEROUS.
          Now let's pick apart your proposed changes, you have issues.

          So you want to use 3-290 watts of panels. That is a total of 870 watts. If you insist on staying with 12 volts which is a huge mistake. It will require a 80 Amp Charge Controller. 870 watts / 12 volts = 72.5 Amps. 45 amps will not cut it. If you move to 24 volts, you will save some serious coins because it only requires 870 watts / 24 volts = 36.25 amps. But there is a catch. Not sure what the Vmp of these 290 watt panels is, but if less than 36 volts (72 cells), you will have to wire them in series to run a 24 volt battery.

          Note on panels. 3 is a PRIME number. That means you only have two configuration options. All in parallel, or all on series. You can get away with prime numbers of 1 and 3. Even 3 can be a problem. If you have to wire the panels in series, the total Voc might exceed the limit of the controller. Having said that the MS Tristar 45 amp MPPT has a 150 Voc which should work with the 3-290 watt panels.

          Batteries. Replace them, they are over due. At 1 Kwh usage is not a lot, and 870 watts of panel power should be able to generate you 2 to 3 Kwh per day in Summer months. You have to be careful to match panels, controller, battery, and Inverter. Batteries have a minimum/maximum charge/discharge requirement. Go outside those limits and you will run into problems. You want to keep the current no lower than C12 on charge, and no greater than C/8 on charge and discharge. Where C = the battery Amp Hour Capacity. So example a 200 AH battery minimum requirement is 200 AH / 12 Hour Rate = 17 amps, and 200 / 8 = 25 amps. Golf cart batteries you can go up to C/5 or 40 amps in the example. That also limits Inverter size. Example a 200 AH golf cart battery at 12 volts is 12 volts x 25 amps = 300 watts. Golf Cart Battery = 12 volts x 40 amps = 480, call it 500 watts.

          You need to change course. Forget 12 volts to start. Based on 1 Kwh per day usage I am going to bump up to 1.5 Kwh per day for you and recommend:.
          1. Panel Wattage = 400 to 600 watts using 2 panels. Try to avoid Prime Numbers and wire in series. 2-250 watt panel would be perfect at 500 watts.
          2. 24 volt @ 200 AH. Use 4-6volt Golf Cart batteries wired in series.
          3. 15 to 25 Amp MPPT Controller depending on panel wattage. 400 watts = 15 amps, 500 watts = 20 amps, 600 watts = 25 amps. No harm in going higher for future expansion if the wallet can take the pain.
          Use the money I just saved you to buy a 24 volt 1000 watt Inverter. If you use high quality Golf Cart battery like Trojan T-105 you can use 2000 watt if needed based on maximum demand.

          Please do your homework and understand wire and cable sizes and how they relate to current, and the safety issues involved. A 24 volt 2000 watt Inverter requires 100 amp of current. One loose connection or undersized wire is a FIRE. 24 volts will not electrocute you, but 100 amps can make one nasty fire and explosion. So please take time to understand what you are asking for.

          Edit Note:

          Since this is a weekend system, what I proposed can easy do 2 Kwh day in Summer months and 1 Kwh/day in Winter.

          Thanks Sunking.
          Would it be worth the investment to go to 4 panels, 2 strings? That would bump me up to a 60 Amp controller I believe. The 60 amp controller has a few advantages, namely it comes with the LCD screen and an ethernet port which may make monitoring things easier. The cost differential isn't all that much, practically the same if you factor in buying the LCD for the 45 amp controller.

          Looks like going to four panels makes the charge rate too high on 24v, which means a 48v battery system would be required for 4 panels.

          Panel stats: https://www.solarworld-usa.com/~/med...m-frame-ds.pdf
          Vmpp 31.4 V
          Voc 39.9 V
          Isc 9.97 A
          Impp 9.33 A

          Appreciate the help!

          Comment

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