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  • Parrallel Batteries On Separate Charge Controllers

    I'm setting up an off-grid system for my neighbors. But they can't afford to install the full system all at once. So I broke the design up into several modules to be installed over time. Each module is essential a fully functional system with a 400W array, 30A Charge Controller, and 12V 230AH battery bank (2 6V in series).

    So my question is-
    Aside from trying to get the separate battery banks to the same voltage when adding modules together, will there be problems with running parallel 12V batteries that each have their own charge controllers?

    Thanks for the input.

  • #2
    Unless you use different inverters and they have different circuits or can be stackable, all the batteries and charge controllers are combined once you connect the batteries to the same inverter. it is bad idea to setup incomplete system, the off grid solar system is not easy to scale.

    Comment


    • #3
      Originally posted by HRRutz View Post
      So my question is-
      Aside from trying to get the separate battery banks to the same voltage when adding modules together, will there be problems with running parallel 12V batteries that each have their own charge controllers?

      Thanks for the input.
      The parallel batteries is bad idea especially combine old and new batteries together.

      Comment


      • #4
        Originally posted by HRRutz View Post
        Aside from trying to get the separate battery banks to the same voltage when adding modules together, will there be problems with running parallel 12V batteries that each have their own charge controllers?
        Paul's replies are correct... let me elaborate:

        When you put the batteries in parallel you have a single battery bank. You can put as many controllers as you wish onto a single battery bank. You can connect as many loads (inverters) as you wish to a single battery bank.

        The plan to build the system by adding modules is a very poor plan. You should always avoid parallel battery banks.... they are unstable and will have a shorter life than a single series string of batteries.

        Further, as you expand the system you should be at a higher system voltage (24 or 48 volts). The 12 volt inverters will not bu usable in a 24 or 48 volt system.

        If your client can't afford the system that will serve their needs, they would be better off adding "modules" as they can afford, but NOT trying to combine them into one system.

        --mapmaker
        ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

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        • #5
          Thank you for your responses.
          I would not be increasing the voltage in this scenario. I would be keeping it a 12V system as it expands, to save them money.

          I have never seen a solar battery bank that wasn't comprised of several series strings in parallel. It seems to me that that's all this is- several series strings in parallel. Even using 2V batteries, we couldn't make a series-only bank big enough for them. The only other way I can imagine is to start with a full size battery bank (which they couldn't afford to do) on only a few panels as we add more panel capacity over time. The battery bank would never get to charge, which doesn't seem healthy for it. And we'd still have to split the panels between several charge controllers.

          It would be one battery bank with one inverter, but each series string would have its own charge controller, and the battery bank would grow over time (I know mixing old and new batteries can be problematic, but it's hard to work with their budget and still get them electricity any time soon). Might each string having its own controller potentially be helpful in keeping an even charge state across the battery bank under these conditions?

          Does anyone have potential solutions?
          Thank you very much for your help.

          Edit-
          Are you saying we shouldnt be adding capacity without essentially having every room of their house on its own individual grid? Living in that house would be a nightmare! Hahaha : )

          Comment


          • #6
            Originally posted by HRRutz View Post
            It would be one battery bank with one inverter, but each series string would have its own charge controller
            <snip>
            Might each string having its own controller potentially be helpful in keeping an even charge state across the battery bank under these conditions?
            If the battery strings are in parallel, then all the charge controllers are also in parallel. Sorry to be so blunt, but if you don't understand this then you shouldn't be designing power systems.

            Originally posted by HRRutz View Post
            Are you saying we shouldnt be adding capacity without essentially having every room of their house on its own individual grid? Living in that house would be a nightmare!
            It will be a worse nightmare if you put all those systems in parallel. Properly designed systems are expensive, but they are, in the long run, MUCH less expensive than improperly designed systems.

            --mapmaker
            ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

            Comment


            • #7
              Originally posted by mapmaker View Post
              If the battery strings are in parallel, then all the charge controllers are also in parallel.
              Can you explain what you mean by this?
              As I've said, my intention had been to have every series string on its own controller, so they would be parallel in the exact same way. Be as blunt as you need to.

              Would a drawing help to clarify?

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              • #8
                Originally posted by HRRutz View Post
                Would a drawing help to clarify?
                Hopefully, yes. Words don't seem to be working. --mapmaker
                ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

                Comment


                • #9
                  20140619_085254.jpg
                  I hope this helps to explain my intentions a little better.
                  It would be nice to go to 24V, but I'm having difficulty talking them into the upfront expense.

                  Comment


                  • #10
                    Originally posted by HRRutz View Post
                    I hope this helps to explain my intentions a little better.
                    The two chargers, the two battery strings, and the load are all in parallel.

                    Basic rule: if A is parallel to B, and B is parallel to C, then A is parallel to C.

                    A, B, and C can be batteries, chargers, loads, or anything else.

                    The system will work, but is very poor design, if for no other reason than parallel batteries are unstable and will be shorter lived than a single series string.

                    Originally posted by HRRutz
                    I have never seen a solar battery bank that wasn't comprised of several series strings in parallel.
                    Then you have never seen a well designed system.

                    recent discussion: http://www.solarpaneltalk.com/showthread.php?15658

                    --mapmaker
                    ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

                    Comment


                    • #11
                      The last one I did was a 24V system with a single 60A Tristar controller and 2 parallel strings of Trojan L16's. I've tried to convince these people to go the same route, but they have a very small budget and wanted to explore other options.
                      I was nervous about this 12V design with separate controllers, which is why I came for advice, but it was the best I could come up with for them for the price. Not ideal, but functional. And it gets them usable power as soon as possible.

                      Comment


                      • #12
                        They'll work just fine, keep the connections tight and clean and maintain the batteries in a healthy state and they'll have a long life. Using multiple charging sources works unless they fight each other, causing one or more to terminate early, that you'll have to watch.

                        Comment


                        • #13
                          Thanks, Willy.

                          Does anyone have Advice or a Reason for not liking this idea, other than just being conditioned to not like parallel strings?

                          I've talked them into upgrading to bigger batteries and a bigger charge controller, so we'll only have 1 string while we have the 1st and 2nd modules installed. Then we'll hopefully move up to 24V when we install the 3rd and 4th. So it's really a mute point.

                          But I had already considered all the arguments given before coming to ask for advice, and none of them seemed to apply in this case.

                          Comment


                          • #14
                            Originally posted by HRRutz View Post
                            It would be one battery bank with one inverter, but each series string would have its own charge controller, and the battery bank would grow over time (I know mixing old and new batteries can be problematic, but it's hard to work with their budget and still get them electricity any time soon).
                            Then they will have a lousy system. As long as you make that clear, you are at least being up-front with them.
                            Might each string having its own controller potentially be helpful in keeping an even charge state across the battery bank under these conditions?
                            No. As others have pointed out, paralleling all strings allows you to control the voltage - but not the current - to each string. And lead acid batteries are better off when you can monitor and control their current. That's why one string is ideal and a few matched strings are still OK (relatively good current sharing.)
                            Does anyone have potential solutions?
                            Expand about every 4-5 years, and replace the entire battery bank when you do it.
                            Are you saying we shouldnt be adding capacity without essentially having every room of their house on its own individual grid? Living in that house would be a nightmare! Hahaha : )
                            ?? No.

                            If you wanted to get technical, then a system where each bank was charged/monitored independently, and discharged in common by one inverter, would be pretty ideal if your goal was to add battery strings at any time and have them all last. However that will tend to be a much less efficient system.

                            Comment


                            • #15
                              Originally posted by paulcheung View Post
                              Unless you use different inverters and they have different circuits or can be stackable, all the batteries and charge controllers are combined once you connect the batteries to the same inverter. it is bad idea to setup incomplete system, the off grid solar system is not easy to scale.
                              ^^^ this.

                              Originally posted by mapmaker View Post
                              Paul's replies are correct... let me elaborate:

                              When you put the batteries in parallel you have a single battery bank. You can put as many controllers as you wish onto a single battery bank. You can connect as many loads (inverters) as you wish to a single battery bank.

                              --mapmaker
                              ^^^ and this.

                              I don't understand why you (OP) are insisting on running the 2 strings parallel into one inverter...?

                              Is it because they can only afford one inverter? Either you budget in a 2nd inverter or the system is not going to work as you are expecting it to.

                              You can "sell" them the benefit of redundancy of a 2nd inverter.
                              If one inverter happens to fail, at least half the system is still providing electricity.

                              What is the load/usage?
                              If lighting is involved, you could run DC LED lighting on one string without the need for an inverter.
                              That would reduce typical inverter losses, making your build more efficient.
                              [CENTER]SunLight @ Night[/CENTER]

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