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  • Batteries in series - dissimilar voltage while charging?

    Thank you in advance for your help.

    Problem: Two 6V batteries in series show different voltages when charging (0.5V difference - 7.56V and 7.06V). Batteries show the same voltage at rest (6.5V). Batteries bubble/gurgle when charging (14.5V @ ~2.2 amps). Battery bank shows ~13.0V when not charging. I'm concerned about the difference in voltage while charging primarily and the bubbling secondary.

    Also, the charge controller is jumping from showing 13.1V (battery voltage resting) up to 14.5V when the PV array is turned on. Shouldn't the charge controller show the voltage state of charge of the batteries and not the voltage it's charging with?

    When the PV is released, the voltage jumps back down to 13V instantly.

    ----

    Batteries: (2) 6v AGM wired in series as a 12V bank.
    Charger: 30A EPEver charge Controller
    Panels: (4) 100W solar panels


    System is isolated / not chassis grounded.

    Was going to independently charge them with a battery charger, but they're already at the same resting voltage. Is one of my batteries bad because it's not the same charging voltage as the other? Is my charge controller overcharging my batteries?


    Thanks again. I'm lost here.

    edit: understood mod note. no site indication was given of why post wasn't working. thought it was on my end. thanks for the help
    Last edited by adrewnaline; 06-12-2019, 08:58 PM.

  • #2
    Hi adrewnaline, welcome to the forum. I'm sorry about the posting problems you had, it looks like it was because of the links and the forum automatically flagged them for moderation. It's an anti-spam measure, not intended to stop informational links like yours. I'll re-post them for the benefit of those trying to help.

    Interesting post and looking forward to hearing what the experts have to say. Again, welcome.

    - Steve

    Batteries: (2) 6V VMAX AGM Batteries wired in series as a 12V bank.
    Charger: EPEver 30A Charge Controller
    Panels: (4) 100W Renogy 12V solar panels
    System is isolated / not chassis grounded.

    Comment


    • #3
      Originally posted by adrewnaline View Post
      Thank you in advance for your help.

      Problem: Two 6V batteries in series show different voltages when charging (0.5V difference - 7.56V and 7.06V). Batteries show the same voltage at rest (6.5V). Batteries bubble/gurgle when charging (14.5V @ ~2.2 amps). Battery bank shows ~13.0V when not charging. I'm concerned about the difference in voltage while charging primarily and the bubbling secondary.

      Also, the charge controller is jumping from showing 13.1V (battery voltage resting) up to 14.5V when the PV array is turned on. Shouldn't the charge controller show the voltage state of charge of the batteries and not the voltage it's charging with?

      When the PV is released, the voltage jumps back down to 13V instantly.

      ----

      Batteries: (2) 6v AGM wired in series as a 12V bank.
      Charger: 30A EPEver charge Controller
      Panels: (4) 100W solar panels


      System is isolated / not chassis grounded.

      Was going to independently charge them with a battery charger, but they're already at the same resting voltage. Is one of my batteries bad because it's not the same charging voltage as the other? Is my charge controller overcharging my batteries?


      Thanks again. I'm lost here.

      MOD Note: A new members first few posts may need to be approved by a Moderator before they become viewable. Reposting the same content will not get this to happen any sooner. Three duplicates of this post have been deleted.
      The CC has no idea what the resting voltage of the battery might be. All it can see is the voltage at the battery terminals while it is charging them. For this reason the CC is configured to control the voltage it applies to the battery bank under various conditions. A measurement at the battery terminals is only an accurate indication of the State of Charge (SOC) of the bank when there has been neither load nor charging current for three or four hours.
      If the batteries are at different SOCs or in different health when put in series, it takes a long time gently charging in series to get a stack of AGM batteries fully charged. With Flooded batteries (FLA) you can just apply a high enough voltage to force current through the fully charged batteries to continue charging the rest. This is called equalizing. Trying to do that with AGMs will instead destroy the already charged batteries.
      Your idea of charging them each separately is a good one, but make sure that your charger has a setting for AGM batteries!
      SunnyBoy 3000 US, 18 BP Solar 175B panels.

      Comment


      • #4
        Originally posted by inetdog View Post
        The CC has no idea what the resting voltage of the battery might be. All it can see is the voltage at the battery terminals while it is charging them. For this reason the CC is configured to control the voltage it applies to the battery bank under various conditions. A measurement at the battery terminals is only an accurate indication of the State of Charge (SOC) of the bank when there has been neither load nor charging current for three or four hours.
        If the batteries are at different SOCs or in different health when put in series, it takes a long time gently charging in series to get a stack of AGM batteries fully charged. With Flooded batteries (FLA) you can just apply a high enough voltage to force current through the fully charged batteries to continue charging the rest. This is called equalizing. Trying to do that with AGMs will instead destroy the already charged batteries.
        Your idea of charging them each separately is a good one, but make sure that your charger has a setting for AGM batteries!
        Thank you so much for your quick reply!

        1) So I should be setting the equalization charge voltage to 0 / off just to be safe that it doesn't try to equalize?

        2) If I go back and measure the batteries in 5 hours or so, and they're exactly the same voltage, but under charging one of them rises instantly about 0.5V above the other, and upon swapping the order of the two batteries in series, the high-voltage and low-voltage batteries are the same, would this suggest one of the batteries is bad?

        3) If they're the same voltage after 5 hours, but if when swapping the batteries, the one that was high is now low when charging, would this suggest anything you've seen before?

        If, after resting overnight, the batteries have different resting voltages, I'll a 6V AGM charger and charge them both up for a while, reconnect in series, and check the voltage difference while charging.

        You're a huge help. Thanks again!
        Last edited by adrewnaline; 06-12-2019, 08:43 PM.

        Comment


        • #5
          SOME FOLKS HAVE some degree of success with battery balancers, HA02 being among the budget freindly alternatives. in 1 of my setups 2 x 12v 200ah battery in series, batt1 = 13.9v, batt 2 = 13.5v, when i hooked up the balancer the batts now read 13.5v and 13.6v respectively, so i think the battery balancer is doing something, only time will tell

          Comment


          • #6
            Just let them settle and they have the same resting voltage but one consistently has higher voltage when charging than the other even when swapping the order in the series.

            Guessing batteries aren't in same service life condition. Going to try and return them and go 12v in parallel instead.

            Comment


            • #7
              Originally posted by adrewnaline View Post
              Going to try and return them and go 12v in parallel instead.
              In cycle service (such as solar), stick with series. Parallel batteries usually charge and discharge with unequal currents, but a series arrangement ensures that the charge/discharge currents are the same, so the energy put into or taken out of each battery is the same.

              Comment


              • #8
                Originally posted by sdold View Post
                In cycle service (such as solar), stick with series. Parallel batteries usually charge and discharge with unequal currents, but a series arrangement ensures that the charge/discharge currents are the same, so the energy put into or taken out of each battery is the same.
                Hmm, interesting, thank you. So series is recommended (two 6v to make 12v), but I'm struggling that the two 6V batteries aren't at the same voltage when charging. Very disappointed and a little frustrated. Sounds like it's a crap shoot to get two 6V batteries that I can wire in series that will both hold the same voltage when charging :-/

                Comment


                • #9
                  Since the internal resistances of two batteries will never be exactly the same, I don't know how important it really is that the voltages be equal when charging. I'm curious about it too but not worried about it since I don't remember the topic coming up or having it be an issue before.

                  Take a look at the thread below when you get a chance, interesting reading on this topic:

                  https://www.solarpaneltalk.com/forum...es-in-parallel

                  Comment


                  • #10
                    Sdold, thank you for that link. Very interesting thread. A couple engineers there are very passionate about not going parallel with AGM or Pb batteries in cycling services. I'll take this expert advise and avoid it.

                    My apologies on beating a dead horse here, but my batteries have a difference of up to half a volt while charging from solar. That seems like quite a large gap, especially when the range of recommended float and charge voltages are less than .5V. As some battery experts call it, getting the right charge voltage per cell is like "balancing on the head of a pin." Having 1/2 V difference between the two 6V batteries while charging just seems too big to ignore.

                    All that said, I do admit I'm a novice and if this isn't considered a problem, maybe I should just not worry about it. I've contacted VMAX but haven't heard back yet.

                    Comment


                    • #11
                      Originally posted by adrewnaline View Post
                      So series is recommended (two 6v to make 12v), but I'm struggling that the two 6V batteries aren't at the same voltage when charging.
                      Remember that the "6V" and "12V" batteries are each comprised of a series connection of "2V" cells. With the FLA, sg of the electrolyte can be measured; in many, individual cell voltages can be as well. I don't think individual measurement is common with AGM; it certainly doesn't look possible on the picture in your link.

                      While certainly not a battery engineer, I remember a common failure mode in FLA in cars was a bad cell; that sounds like a possibility. Perhaps someone here can tell you how to do further diagnosis.

                      Comment


                      • #12
                        I have stated several times that an infrared heat gun will show a bad cell in an sealed battery or for that matter any battery, the failing/failed cell will be noticeably different in temperature than the good cells. The infrared camera shows a failed very graphically, however this can be done with a inexpensive spot temperature gun. Heat caused by a shorted cell will be at the tops of the plates in the cells, maybe about 1-1/12 inches down from the top of the battery case and around the terminal area. Battery manufacturers are using this technique. Steve Higgins at Rolls -Surette has suggested that diagnostic method to me but I allready knew.

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