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  • Intermittent use system design guides for remote weekend get away.

    Hello all, first post ot this forum.

    I have been tackling a PV system that is a little backwards from the standard design approach.

    I have a weekend getaway/ hunting cabin in Southern Ohio. My needs are to have power while there for 2 to 5 days, then I may be gone for weeks on end. What I am doing now is I haul with me a 80Ah D-S marine battery and hook it up to a 400W inverter to run the lights. This will run 1 to 2 CFL for 3 days and then I haul it home and charge on my home battery charger hooked to the grid.

    I would like to set up a battery bank that will provide the power I need to run my lights and a few other items for 3 to 5 days. Then when I am gone let a PV panel recharge them over X days. This could be 5 days or 14, I have not decided. During this time there will be no load.

    Traditional design covers designing the battery bank. (Ah * 5 days vs. Ah capacity of the batteries (including derates, and discharge %))

    Where I am at a loss is I do not need an array that needs to provide for direct power consumption. i.e. an array that will provide 1000 watts.

    I am interested in the thin film amorphous PV attached to a MPPT regulator to capture the higher volts/watts supply the 12V or 24V battery bank. Another option is a single 20w 30W or so panel that can directly supply a PWM regulator or so.

    I am not sure how to calculate recharge time, or what combo to put in place.

    Are there any guides on this type of system?

    I need to keep install costs down as there is a potential for theft if the components are too nice.

    Thank you for your input.

  • #2
    Two issues:

    You want to use up to 5 days at a time. Two days is peice of cake. Traditionally you are correct 5 days reserve, but in practice is only 2.5 days of usable capacity so as not to go below 50% DOD.

    Minimum charge current requirement is C/12. Assuming a 12 volt 100 AH battery if you use PWM controller requires a 150 watt panel and if using MPPT requires a 100 watt panel to produce 8.3 amps or C/12 on a 100 AH battery.

    So lets make this simple. Determine watt hours you need in a single day, and multiply by 5, then divide by 12 volts to get required Amp Hours @ 12 volts. Then determine minimum panel wattage to get you up to C/12 charge rate.

    The let me know where this cabin is so I can steal everything. Should be easy to find with all the panels.

    If it were me I would do one of two things depending on daily requirement. If I could carry a large enough AGM battery I would size it for 8 to 10 day reserve capacity and charge it up at home. Or size it to 3 day capacity and use a small generator to charge up daily and take it home with me in case my friends come along when I am gone. But that is just me. Do what you want and can afford to loose.
    MSEE, PE

    Comment


    • #3
      Assuming you are happy with the size of the 80ah battery, and are just looking for it to get charged over 5 days that you are gone, and you won't be using more than 1/2 of the battery (40ah), you'll need 40ah/5 days of charging, so 8ah a day needs to be generated. You didn't say if it is used year round, so I'll assume worst case of 2 sun hours for winter, 8ah/2 hours = 4 amps / .67 system inefficencies = 6A of solar needed. That's about 120W of solar.

      Going the other way, 2 13W CFL lights for 3 hours a day = 78wh a day. 78wh / 2 sun hours = 39W / .67 = 58W of solar can provide enough solar for the lights, add another 5% for the inverter (or switch over to DC lights), and 60W of solar will provide enough power in a day. If you assume that you will be gone at least twice as long as you are there, 78wh / 12V = 6.5ah of that battery is being used each day you are there, so theoretically, it'll take 6 days being at the cabin to get it to 50% drained. So, with a bunch of assumptions, a 30W panel can charge in 2 days what is used in 1 day. Put a little Morningstar SunGuard 4.5A charge controller, and you should be in much better shape than you are now. Bring a volt meter, and if the battery starts to get low because of no sun when you are there, go home early, the weather stinks anyways.
      Solar Queen
      altE Store

      Comment


      • #4
        I will admit I am a little lost in understanding the C/12 charge rate statement.

        More specifically I am looking at installing 2 6V 220 Ah Lead Acid batteries in series. For a capacity of 110 Ah. to be used during my stay. That part I have down. i.e. each 13w light consumes ~ 1.25A/hr (12w/12V /.87 inverter eff) thus I have a budget of 22Ah/day on a 5 day stay (110/5) and can have 4 lights on for 5 1/2 hours.

        I still am a little lost on the re-charge set up. I will admit I do not understanding the C/12 charge rate statement. Is that a min. charge rate for battery life, to get them to charge at all? I assume the 12 is based on the 12 V battery. What is C? The Ah rating of the battery or the # of Amps needed to be re-charged, something else?

        I am probally thinking about this all wrong so please correct me or point me to where I can learn.

        I am under the understanding as long as I can get more than 15V then the battery will charge. The "pressure" is there but the Amps "Volume" is variable. So when I leave and I cionnect the batteries to a 30 watt panel with a 15V charge regulator, it will provide (assuming no losses) 2 amps. My location has 4 sun hours. 2 amps for 4 hours/day will charge the batteries 8Ah/day and while I am away it would take ~14 days to recover the 110Ah of use.

        Is this a wrong way of thinking? My understanding is slower charging rates are better for batteries.

        Thank you for helping me to learn.

        Comment


        • #5
          Originally posted by Amy@altE View Post
          Assuming you are happy with the size of the 80ah battery, and are just looking for it to get charged over 5 days that you are gone, and you won't be using more than 1/2 of the battery (40ah), you'll need 40ah/5 days of charging, so 8ah a day needs to be generated. You didn't say if it is used year round, so I'll assume worst case of 2 sun hours for winter, 8ah/2 hours = 4 amps / .67 system inefficencies = 6A of solar needed. That's about 120W of solar.
          Usally we will run out there for a weekend and not be back for a month or more. During hunting season (1wk of Dec.) I'll be out there 8days then not back until April. (Mid-week I go and charge the battery) Time between visits is very long compared to time there. Thus charging over 30 to 60 days would be fine in the winter and 14 to 20 days in the summer. Summer here is 4.5 to 5 sun hours.

          I am ignoring any solar generation while I am there for simplicity, Anything produced while there is margin.

          One system I am looking has: Electrical Specs STC
          Maximum Power (Pmax) 136 Watt
          Nominal Operating Voltage 24 Volt
          Maximum Power Voltage (Vmpp) 33V +-5%
          Maximum Power Current (Impp) 4.1A
          Open Circuit Voltage (Voc) 46.2V
          Short Circuit Current (Isc) 5.1A
          Maximum System Voltage 600V

          Pricewise this unit looks to be decent though a little large dimensionally.

          The controllers I am looking have a statement in the specs that the peek open circuit Voltage of the solar panel be 18V to 24V for a 12V system or 36V-48V for a 24V system. This is a concern as this controller and panel do not seem to match-up. The further complicate things for me there is a YouTube video of this panel showing it producing 53V in shaded sun... (My ignorance is killing me here)

          This panel may be way too big but $/watt is seemed a good trade. I just do not want it to drive me to a $400+ charge controller. Ideally I want to stay with the $15 to $50 controllers capable of 10A to 30A's.

          Comment


          • #6
            Originally posted by Jeff-oh View Post
            Usally we will run out there for a weekend and not be back for a month or more. During hunting season (1wk of Dec.) I'll be out there 8days then not back until April. (Mid-week I go and charge the battery) Time between visits is very long compared to time there. Thus charging over 30 to 60 days would be fine in the winter and 14 to 20 days in the summer. Summer here is 4.5 to 5 sun hours.
            You are killing your batteries and will have junk when you want them - this is NOT an application for solar.
            [SIGPIC][/SIGPIC]

            Comment


            • #7
              Put your money towards a nice and quiet 1000 watt honda or yamaha generator and charge the battery you have. Smart money and discrete. You will be amazed how little you really need to run the generator.

              Comment


              • #8
                Originally posted by Jeff-oh View Post
                I will admit I am a little lost in understanding the C/12 charge rate statement.
                Pretty simple math C would be the battery Amp Hour rating , and 12 is the discharge rate in hours. So for a 100 AH battery C/12 = 100 AH / 12 H = 8.25 amps

                Originally posted by Jeff-oh View Post
                More specifically I am looking at installing 2 6V 220 Ah Lead Acid batteries in series. For a capacity of 110 Ah. to be used during my stay. That part I have down. i.e. each 13w light consumes ~ 1.25A/hr (12w/12V /.87 inverter eff) thus I have a budget of 22Ah/day on a 5 day stay (110/5) and can have 4 lights on for 5 1/2 hours.
                Well 2 6-volt 220 AH batteries in series is 12 volts @ 220 AH. Forget AH and use Watt Houts

                Originally posted by Jeff-oh View Post
                I still am a little lost on the re-charge set up. I will admit I do not understanding the C/12 charge rate statement. Is that a min. charge rate for battery life, to get them to charge at all? I assume the 12 is based on the 12 V battery. What is C? The Ah rating of the battery or the # of Amps needed to be re-charged, something else?
                Already explained C/12. It is the minimum charge requirement for lead acid batteries. On a 220 AH battery would mean 220 AH / 12 hours = 18.3 amps minimum . To do that with solar if you used PWM would require 330 watts of solar panel and 220 watt with MPPT. However that may or may not work because the panel wattage depends on what type of charge controller, how many watt hours you will use in a day, and the Sun Hours you receive.
                MSEE, PE

                Comment


                • #9
                  How about looking at it from this angle using your existing equipment without even thinking solar yet:

                  Without taking into account peukert and losses, we can do the napkin-calc:

                  Your 80ah battery can be calculated to supply 960 Wh. 80ah * 12v.
                  Since you don't want to use more than half of the battery capacity to get any decent life from it, we cut that value in half: 480 watthours usable. But let's cut off at least 10% of that to ballpark losses. That's really 432 watthours usable.

                  Plan for worst case. Does the total wattage of your existing CFL's when counted over the hours of use you put them to exceed 432? If so, you need to conserve in either wattage, or time used.

                  The C/12 requirement means dividing the rated ah capacity of the deepcycle battery by 12. The reason for this is that for flooded, you need a minimum amount of current to prevent electrolyte stratification. So that means 80 / 12 = 6.6666 amps minimum to charge regardless of how many days you have to accomplish charging. The absolute max for flooded would be C/8, ie 10A max.

                  Question: What is the total wattage of the CFL's you use, and for how many hours do you keep them on before recharging? What is the make / model of the battery you are using now? Is it flooded or sealed agm? Also, what is the charger you are using? Some automotive type chargers are VERY agressive, and can actually cut down the cycle life on healthy batteries.

                  If we get this fine-tuned, then perhaps we can move on to solar.

                  Comment


                  • #10
                    Ditch the inverter and use 2.5w, 12V LED's

                    A couple of these will be less power than the CFL & inverter, and direct the lighting where you want it.
                    LED_module2.2W.jpg
                    https://www.wiredco.com/10_LED_SMT_S...olwhiteled.htm

                    I've used a bunch of them and they are great. Warm white version also, but less bright.

                    As to solar, there are 2 equations to deal with.
                    a) Recharge must be accomplished within 3 days, or the battery begins to sulfate. (AGM batteries less so)
                    b) Minimum charge rate to keep the electrolyte mixed up properly. This is the C/12 rate. AGM can ignore this.

                    So, first you want to calculate out your load in watt hours, and then that gives you the size of battery you need, Then you figure out the solar to charge it.

                    For the simplest thing, get an AGM sealed deep cycle, a battery charger, and a small honda EU1000 inverter generator (or clone). Anything more gets expensive and complicated fast, and is less adaptable.
                    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                    solar: http://tinyurl.com/LMR-Solar
                    gen: http://tinyurl.com/LMR-Lister

                    Comment


                    • #11
                      Originally posted by Mike90250 View Post
                      Ditch the inverter and use 2.5w, 12V LED's

                      A couple of these will be less power than the CFL & inverter, and direct the lighting where you want it.
                      [ATTACH=CONFIG]5220[/ATTACH]
                      https://www.wiredco.com/10_LED_SMT_S...olwhiteled.htm

                      I've used a bunch of them and they are great. Warm white version also, but less bright.

                      As to solar, there are 2 equations to deal with.
                      a) Recharge must be accomplished within 3 days, or the battery begins to sulfate. (AGM batteries less so)
                      b) Minimum charge rate to keep the electrolyte mixed up properly. This is the C/12 rate. AGM can ignore this.

                      So, first you want to calculate out your load in watt hours, and then that gives you the size of battery you need, Then you figure out the solar to charge it.

                      For the simplest thing, get an AGM sealed deep cycle, a battery charger, and a small honda EU1000 inverter generator (or clone). Anything more gets expensive and complicated fast, and is less adaptable.
                      The only issue I see with those LED lamps is that now you need to find a "base" to plug it into and then wire that base to the DC distribution system.

                      I have an RV and just replaced about 24 of those incandescent 12volt 6watt lamps with LED 2watt lamps that have the same 1156 base found in most vehicles. Made it real easy to lower my wattage without having to rewire the fixtures.

                      Comment


                      • #12
                        The bi-pin lamps are replacements for 10-50w halogen bi-pin lamps. They have great directionality, and give more useable light than the 1156 base bulbs. The disc lamps also have a driver circuit, which accommodates AC or DC, 9-30V which handles 12 or 24V installs, plus replacing the halogen bulbs
                        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                        solar: http://tinyurl.com/LMR-Solar
                        gen: http://tinyurl.com/LMR-Lister

                        Comment


                        • #13
                          Originally posted by Mike90250 View Post
                          The bi-pin lamps are replacements for 10-50w halogen bi-pin lamps. They have great directionality, and give more useable light than the 1156 base bulbs. The disc lamps also have a driver circuit, which accommodates AC or DC, 9-30V which handles 12 or 24V installs, plus replacing the halogen bulbs
                          Nice. I did not know that about those LED lamps. Thanks

                          Comment

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