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  • Daytime use solar with minimal batteries?

    Hello All,

    I have found this site to be a fantastic learning resource. Thank you to all who make it possible.

    I do have a basic understanding of system sizing (panels, CC & batteries) based on anticipated loads.

    I lived for a few years in a location where the local utility wanted over $20k to run a water line to the home. Unless I wanted to spend $20k I had to adjust my lifestyle to live with what water I could collect with my rainwater system. I did just that. Modified lifestyle and habits to work around available financial and natural resources.

    I was contemplating an off grid project on some remote land in AZ for seasonal use. As batteries are one of the most expensive and shortest lived components in a solar system I got to wondering if there is some reasonable method of installing minimal batteries and adjusting lifestyle/habits to operate all large loads when the solar is producing and limiting loads when the solar is not providing power.

    With panels in the 30-50 cent a watt price range I could afford to panel enough to run some power tools etc mid day.

    My household loads are pretty limited. LED TV & lights, RV water pump and cell charger. Refrigeration will be with a very well insulated cold plate system. It will run the compressor only when the solar is producing.

    Perhaps a small battery bank of whatever chemistry battery that allows for the highest charge rate?

    Another (crazy?) thought I had was to build a small off grid / battery / inverter system and feed the AC input of a grid tie inverter with the big solar array feeding that inverter. Nuts or a reasonable idea?

    I simply want to use the vast majority of my power when the sun is shining and run the smallest battery bank that I can manage.

    Your constructive input is much appreciated.

    S.

  • #2

    OK. First off the "I want to fool a big grid tie inverter with a little tiny inverter" won't work for several reasons. That's AC coupling and you need a large inverter with a battery; each grid tie inverter you add must be lower power than the battery inverter. You also need relays and some intelligent control that is not trivial.

    But to your original question (small battery bank/large solar array) that is doable with a few caveats:

    1) You have to be VERY careful to not draw down the bank past 50%. A good battery monitor that can keep track of amp-hours is critical here.

    2) Most batteries are limited in how much power they can accept. So you need a charge controller that can limit current based on BATTERY (not output) current. The Whizbang Jr in combination with the Midnite Classic can do this, for example. This way you can limit charge rate, but have your MPPT provide more current for your refrigerator during the day (for example.)

    3) LiFePO4 can accept at rates up to C/2, AGM to C/4, flooded to C/8. However you are probably better off with 400ah of flooded cells vs 100ah of LiFePO4. The charge rate will be the same in both cases - but lead acid will be cheaper and store far more energy.

    Comment


    • #3
      Originally posted by PNW_Steve View Post

      Another (crazy?) thought I had was to build a small off grid / battery / inverter system and feed the AC input of a grid tie inverter with the big solar array feeding that inverter. Nuts or a reasonable idea?
      This is called AC coupling and it only works with Bimodal (grid tie with battery backup ) NOT off grid systems. Further the bimodal system HAS to be LARGER than the non-bimodal inverter. It has to be able to take the full output of the non-bimodal inverter.
      On top of that the bimodal inverter needs to have a battery attached that is capable of taking the full charge from the non-bimodal inverter as well.

      SO your idea is basically nuts as stated (dont worry lots of people have the same idea).

      Getting an AC coupled system to work is generally more costly and more complicated than just installing a bimodal inverter in the first place and there are few advantages for the added cost and complication.

      For your want you are probably better off just installing bimodal system with AGM or Flooded batteries. If the system is just for occasional backup, AGM though more costly often have a better shelf life and are lower maintenance as well as higher charge rate.
      OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

      Comment


      • #4
        While this idea is frowned upon, I live with only a car battery for 5 months a year. That is because I don't want to buy any battery, this one is out of a vehicle I don't bring with me and would be destroyed if I left it home. Even with a small charging panel. I have 2,500W of panels, chest fridge only operates in daylight hours, battery never gets discharged below 12.5V, Also have a dishwasher with heated dry. There is a second battery which barely functions as it wouldn't start a car 4 years ago. That house battery operates the lights, tv and DVD. The "fridge battery" must stay fully charged so a medical device can run at night and fridge has to start first thing in the morning. And all the basic functions operate in the most cloudy rainy day. And if it is any kind of normal day, I have all the hot water I need. If I ever get my boat in the water I'll have a good second battery for the house. Hate dragging batteries back and fourth. Might go lithium. Minimal battery is an interesting concept that no one cares about. I'd say don't try this at home, but that is highly unlikely as it takes a lot of custom controls to keep it from destroying things. It is amazing what can be done with some lifestyle changes. I asked my wife once if the dishes in the dishwasher were clean. Thought they might have been the ones done the day before. She said No, I'm waiting for the sun.

        Comment


        • #5
          Originally posted by jflorey2 View Post

          But to your original question (small battery bank/large solar array) that is doable with a few caveats:

          1) You have to be VERY careful to not draw down the bank past 50%. A good battery monitor that can keep track of amp-hours is critical here.

          2) Most batteries are limited in how much power they can accept. So you need a charge controller that can limit current based on BATTERY (not output) current. The Whizbang Jr in combination with the Midnite Classic can do this, for example. This way you can limit charge rate, but have your MPPT provide more current for your refrigerator during the day (for example.)

          3) LiFePO4 can accept at rates up to C/2, AGM to C/4, flooded to C/8. However you are probably better off with 400ah of flooded cells vs 100ah of LiFePO4. The charge rate will be the same in both cases - but lead acid will be cheaper and store far more energy.

          Thank you All for the input.

          #2 Above looks like a possibility.

          I thought that I understood until I pulled up the manual for the Classic. I expected to see:

          PV-In
          Battery-Out
          Load-Out

          Apparently the Classic does not have a "Load" connection like I have seen on other charge controllers.

          Without a load connection, how do I limit charge current without limiting load current?

          Comment


          • #6
            Originally posted by PNPmacnab View Post
            While this idea is frowned upon, I live with only a car battery for 5 months a year. That is because I don't want to buy any battery, this one is out of a vehicle I don't bring with me and would be destroyed if I left it home. Even with a small charging panel. I have 2,500W of panels, chest fridge only operates in daylight hours, battery never gets discharged below 12.5V, Also have a dishwasher with heated dry. There is a second battery which barely functions as it wouldn't start a car 4 years ago. That house battery operates the lights, tv and DVD. The "fridge battery" must stay fully charged so a medical device can run at night and fridge has to start first thing in the morning. And all the basic functions operate in the most cloudy rainy day. And if it is any kind of normal day, I have all the hot water I need. If I ever get my boat in the water I'll have a good second battery for the house. Hate dragging batteries back and fourth. Might go lithium. Minimal battery is an interesting concept that no one cares about. I'd say don't try this at home, but that is highly unlikely as it takes a lot of custom controls to keep it from destroying things. It is amazing what can be done with some lifestyle changes. I asked my wife once if the dishes in the dishwasher were clean. Thought they might have been the ones done the day before. She said No, I'm waiting for the sun.
            Thank you.

            It sounds like you are doing something similar to what I am considering. Could you give a little more detail on how you take advantage of 2500 watts of panels without boiling your battery?

            Comment


            • #7
              First off, I am in a terrible location. My panels sit under trees. Daily cleaning of acorn shells off them is necessary and they lie almost flat. 600W is probably the most you could get out of them as some portion is always heavily shaded and improper angel. The battery never gets much of a discharge and it is pretty hard to get a lot of current into a battery that is not deeply discharged and the voltage is limited. The electronics is custom built (approved by Undertaker Labs) and current limited to an extent and they operate different panel strings. The highest current I've seen out of that and the rest of the system is 22A maximum for a short time when the refrigerator first starts. Currents are allowed to kick up when battery drops below float. Loads are very minimal and I use power directly from the panels for high loads. A lot of people don't think you can run a dishwasher with small solar, but it is less than 100W and 100WH for 1 1/2 hour wash cycle if the heating element is excluded. With extra insulation and hot water to begin with, 200W of raw panel power does the job nicely. I only need the panels for the bad days when I'm happy to get 70W out of the entire system, live in the cloudy Northeast..

              Comment


              • #8
                The Midnight Classic is an advanced charge controller. It also has a add-on module called Whiz-Bang-Jr, which is a fancy current sensor. It attaches to the battery terminal, and will allow the Classic to measure Battery Current, and that will allow the classic to allow a massive PV array and small battery bank to co-exist without fire.
                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


                • #9
                  Originally posted by PNW_Steve View Post
                  I thought that I understood until I pulled up the manual for the Classic. I expected to see:

                  PV-In
                  Battery-Out
                  Load-Out

                  Apparently the Classic does not have a "Load" connection like I have seen on other charge controllers.

                  Without a load connection, how do I limit charge current without limiting load current?
                  Here's how it works:

                  You connect everything to the Battery connections. The controller will not output more current than it can thermally (i.e. it will limit its output to prevent damage.) You can also set a lower output limit. Let's say we set that to 50 amps. Now it will never output more than 50 amps.

                  Now you add the Whizbang Jr to the mix that senses only battery current. Now there is a SECOND current limit that is for the batteries. Let's say you set that to 30 amps. The charge controller will never output more than 50 amps total (per the controller setting) and it will never output more than 30 amps into the battery. But that means you can draw up to 20 additional amps from the output terminals, because the overall limit is 50 amps.

                  Your next question is "how do I enable that?" and I don't know, because that's in a new SW release that I don't have and the manuals on line are for an older release. I will likely see the Midnite Solar folks at SPI in a few weeks; I will ask them then if there's a manual update coming.

                  Comment


                  • #10
                    Originally posted by jflorey2 View Post
                    Here's how it works:

                    You connect everything to the Battery connections. The controller will not output more current than it can thermally (i.e. it will limit its output to prevent damage.) You can also set a lower output limit. Let's say we set that to 50 amps. Now it will never output more than 50 amps.

                    Now you add the Whizbang Jr to the mix that senses only battery current. Now there is a SECOND current limit that is for the batteries. Let's say you set that to 30 amps. The charge controller will never output more than 50 amps total (per the controller setting) and it will never output more than 30 amps into the battery. But that means you can draw up to 20 additional amps from the output terminals, because the overall limit is 50 amps.

                    Your next question is "how do I enable that?" and I don't know, because that's in a new SW release that I don't have and the manuals on line are for an older release. I will likely see the Midnite Solar folks at SPI in a few weeks; I will ask them then if there's a manual update coming.
                    THANK YOU!!!!!!!

                    That is exactly what I was looking for. Looks like a Midnight Classic and Whizbang Jr. are the answer. I have to look a little closer at the Whizbang. Will it replace the functionality of my Trimetric TM-2030?

                    Comment


                    • #11
                      Originally posted by PNW_Steve View Post
                      THANK YOU!!!!!!!

                      That is exactly what I was looking for. Looks like a Midnight Classic and Whizbang Jr. are the answer. I have to look a little closer at the Whizbang. Will it replace the functionality of my Trimetric TM-2030?
                      It will replace the basic functionality of the Trimetric, namely coulomb counting.

                      If you do set out on this path, I'd strongly advise calling Midnite first, explaining exactly what you want to do and then making sure they can supply a charge controller with a SW version that supports this. It would suck to get a great deal on a charge controller only to find out that you need software version 3.8.9.21.2 and so you have to send it back to the factory.

                      Comment


                      • #12
                        Jflorey,

                        I just got off of the phone with Midnight and the get asked "what software version are you referring to?" .

                        Do you know the software version that supports this function?

                        Thanks again.

                        S.

                        Comment


                        • #13
                          Sorry,

                          I missed your reply as I was typing my next question.

                          Is ver. 3.8.9.21.2 the release that supports that function?

                          Thanks.

                          S.

                          Comment


                          • #14
                            Originally posted by PNW_Steve View Post
                            Is ver. 3.8.9.21.2 the release that supports that function?
                            No, I just made that up as a suitably ridiculous version number. I don't know what it is, but here's what Matrix had to say in a different thread:

                            =============
                            The Midnite Solar Classic has a little known internal programing setting (not the one in the menu) that will limit charge amps only, but will not limit amps available for loads. It is a different setting than the Limit output amps setting in the local app or menu. It does require a WhizBang Jr though and the Classic must have firmware version 2096 or newer.
                            =============

                            Comment


                            • #15
                              Originally posted by PNW_Steve View Post
                              Sorry,

                              I missed your reply as I was typing my next question.

                              Is ver. 3.8.9.21.2 the release that supports that function?

                              Thanks.

                              S.
                              The software has updates that a user with win 10 can install. The WBJr has been supported for at least 4 years, so all the firmware has supported it.
                              For midnight support, they have a user forum at http://midniteftp.com/forum/index.php for specific info.
                              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

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