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  • Mppt regulator senses higher voltage when charging

    Hi there,

    After upgrading my system to a higher power panels and higher capacity bank I have found a problem with my new mppt regulator.

    When charging the batteries the mppt recharger shows a higher voltage than the real one.
    For example while charging at 80 amps the difference between the battery bank and the mppt recharger is 0.5 volts.
    Th difference is reduced when the amps go down.
    The mppt recharger shows the the same voltage as the battery bank when not charging.

    I checked the connections and I made sure the cable section is the one given from the manual.
    I am using LIFEPO4 batteries so voltage accuracy is very important.

    Thx so much

  • #2
    Real simple sounds like your cables are too small. You are experiencing excessive voltage drop on your wiring between the controller output and battery term post. If you were to measure the voltage at the output terminals on your controller, and then on your battery term post, you will note the voltage at the controller is higher than batteries with high charge currents.

    However this is just a power loss issue, and assuming the current does not exceed the limits of the wiring is not an issue for the batteries. As the batteries charge up and get near full charge the current will taper down towards Zero and the voltae will be roughly be equal.

    There are two ways to counter this:

    1. Most likely the only option available to you is to increase the wire size and make it as short as possible. The goal is to keep the voltage drop to less than 1%. At 80 amps the minimum wire size is #6 AWG copper. At 80 amps with 1% loss can only be ran roughly 1 foot at 12 volt battery, 2 feet @ 24 volts, and 4 feet @ 48 volt battery.

    2. The second method is not likely an option is the best solution. Control voltage drop to 1% as above, plus use Remote Voltage Sense Leads on your controller. The remote sense leads do not use any current and read the evoltae directly on the batteries. Trouble is I beet your controller does not have remote sense lead input.

    So firs thing is be sure you have at least #6 AWG wire from your controller to your battery for safety. Second if you want to limit voltage and power loss to 1% or less you need to determine your battery voltage (12, 24, or 48 volts), and the one-way cable distance from Controller to Battery and size it properly.

    Otherwise if you have at least 6 AWG, it is safe, and th evoltae drop is not a Real Issue for the batteries. It just means you are burning a lot of power off as waste heat on your wiring and takin glonger to recharge than it should take from a poor design and workmanship.

    Get your volt meter out and get to work. .
    MSEE, PE

    Comment


    • #3
      Thx so much for the info,
      Can u please tell me the cable section in mm?
      Thx so much

      Comment


      • #4
        That would be 16mm cross section, but if the length is longer than 1m you will still have losses . I know as I'm using 16mm for a 48v system and when the charger pushes 30a+ I get 1.5v loss from charger to the buss bar and it's only 1.5m .

        Comment


        • #5
          Dear Raul,

          I am using 6,5 mm cross section and length is 1.8 meters.
          I believe I can move the recharger closer to the batteries shortening the cables down to 60 cm.
          If I rearrange the batteries I can go down to 30 cm.

          Which option would u use?
          What about the section of the cable?
          Consider the cable I am using is the max section i can fit inside the mppt regulator.
          Thx soooo much

          Comment


          • #6
            It can't be . If you got a genuine mppt rated for 80a, the terminals should take atleast 25mm cross section unless is a cheap con Chinese .
            I would have no less than 16 and as close as posible, 60 cm sounds ok.

            Comment


            • #7
              Raul what do you mean for cross section?
              I am using 35 square mm. So the diameter is 6,5 mm.
              It's from Studer.... One of the best in the European market today.
              If this procedure doesn't work can U tell or suggest another brand that works better?
              Thx

              Comment


              • #8
                Originally posted by Barba View Post
                Raul what do you mean for cross section?
                I am using 35 square mm. So the diameter is 6,5 mm.
                It's from Studer.... One of the best in the European market today.
                If this procedure doesn't work can U tell or suggest another brand that works better?
                Thx

                Cross section is the total area of wire section , does not relate to diameter . A 35mm square cable will have a diameter of approximate 9mm and would be atleast awg2 . My battery interconnects are made of 35mm sqare cable and I can tell you for sure if your cable is about 6.5 mm in diameter it will barely be 16mm sqare . I got 25mm jump leads that are muck thicker than 6.5 mm diameter.
                I know studded very well and have good reputation here in Europe for grid application .But for that money I Would go for Sunny Island or outback. If your inverter terminals takes 35mm then use that.

                Comment


                • #9
                  I apologise but I made a mistake the cable I am using is 35 square mm the diameter is the one u say.
                  The inverter Studer is using 50 square mm and there is no loss of voltage there even at 80 amps.
                  i was thinking to connect the recharger directly to the inverter instead the battery, what do u think?

                  Comment


                  • #10
                    No . Connect the charger , inverter and battery bank to a common buss bar . That way you can keep 50mm from inverter to buss Barr , 50mm batteries to buss bar and 35mm charger to buss bar . Your bank interconects have to be minimum 50 since you use that for the load.

                    Comment


                    • #11
                      Thx Raul this exactly what I did and that is how I get the problem.
                      Do u think shortening the cable length is the solution to my problem?

                      Comment


                      • #12
                        Shortening the cable will help . I will also be looking at the terminations . A minimum 8 ton crimp tool should be used to do those lugs properly . Nothing less than high quality multi strand copper thinned low resistance cable also will help keep the loss down .

                        Comment


                        • #13
                          Hi Raul,
                          Where can I buy the multi strand copper thinned low resistance copper cable?
                          Can u buy online?
                          Thx

                          Comment


                          • #14
                            Originally posted by Raul View Post
                            Nothing less than high quality multi strand copper thinned low resistance cable also will help keep the loss down .
                            That is a myth if you are referring Flexible copper cable.

                            The only thing Flexible Standing with Class C, D, G, H, K, and M buys you is as the name implies, Flexibility. There are only two advantages Flexible stranding offers:

                            1. Applications where tight bends are encountered in raceways.
                            2. Applications that subject the wire to repetitive motion like robotics,, cranes, elevators, mining, rotating equipment, doors, etc..

                            There are some significant disadvantages everyone has to consider.

                            1. Low mechanical strength.
                            2. More susceptible to corrosion.
                            3. Poor choice for outdoor use
                            4. Significantly more expensive.
                            5. Termination hardware is more expensive and requires special tooling and training to terminate correctly.

                            All solar installations are fixed. Selecting a Flexible cable stranding for 99% of the installation is a poor choice as there is no advantage in doing so. All it does is significantly raise cost, and expose you to unnecessary risk and premature failure.
                            MSEE, PE

                            Comment


                            • #15
                              Derek , what would you suggest to be used from CC to the buss bar then? I used heavy duty jump leads and made my own terminations. That way I know I got the best quality copper conductor with very low resistance and good insulation. Another option I guess could be welding wire.
                              We have over here ( meter tails ) in 25 and 50mm rigid copper strands that is used from utility fuse box to meter .

                              Comment

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