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Large Voltage Sag on Battery Bank

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  • Large Voltage Sag on Battery Bank

    I was sold these batteries by a local forklift service center as "Trojan Black Box". They do not have trojan badges on them and I was told they are 210ah 6v flooded batteries. I have 4 batteries wired in series-parallel giving me a 420Ah 12v bank for use as a house battery on a camper.

    I have had them in service for ~3 mo and I have started to notice a lot of Voltage Sag. During these 3 months, the batteries have been charged by solar panels at 14.4v and by the alternator to a similar voltage while driving. What recently got my attention was that I saw the battery voltage drop to ~12v under a small 20A load (C/20) shortly after coming off being fully charged. I thought that was far too dramatic of a drop and started investigating.

    I increased the charge voltage to 14.8v to try and make sure they were getting a full charge. Here is a picture with the up to date hydrometer numbers and voltages. the lowest is a 1.256 and the highest is a 1.276. They are very consistent between cells of the same battery. Voltages are 7.5v &7.3v for one series string and 7.6v and 7.2v on the other string.

    When doing my hydrometer readings I also noted that one series string has significantly more black build up on the underside of the caps. The other string has hardly any. Not sure if this gives any clues as to what may be happening during charge or discharge.

    I have also noticed when bubbling at 14.8v that I get very large bubbles. It is not a constant fizzing sound but more like a big bubble just got released and rushed to the top of the battery. I have also had a small amount of electrolyte come out of 2 battery cells during this charging. It is almost always from the same cells which are on the string with the black deposits on the caps, The fluid level appears to be correct at just below the well.

    Hoping you all can help enlighten me on what has happened and hopefully reclaim some of my capacity.

    Please let me know what other investigation I can do to help troubleshoot!

    Attached Files

  • #2
    To start with with your batteries are in desperate need of Equalizing. You are looking at the wrong voltages under charge. Voltage is only accurate on Open Circuit fully rested batteries. In other words disconnected and allowed to sit for 24 hours at room temps. Once rested a properly Equalized battery at full charge will be 6.3 to 6.4 volts with no greater than 0.05 volts spread in voltage and Specific gravity at 1.277 +/- .003 points. You are no where remotely close to being equalized as you have a voltage spread of 0.4 volts and SG spread of .03. You are off by a factor of 10.

    Where are you measuring this voltage sag? Only valid measurement point is directly on the battery Term Post. Any other place is invalid. Example if you measure 12.5 volts on a load battery term post, and say 12 volts at the input of the Inverter is telling you have poor wiring and there is nothing wrong with the battery. All operator error.
    MSEE, PE


    • #3
      Sorry for the slow reply. I have been very caught up this week.

      The only electrical source I have avalible is my solar but I will set it to equalize at 16.2v as suggested by Trojan tech support. Total time I'll be able to equalize may be somewhat short depending on how much sun we get. I assume I can do this multiple times over a few days to get the desired effect? Also what should the current be when equalizing? if I use the solar panels they are charging and equalizing at the same time so current could be ~15-20A

      I havenot been able to test the batteries rested as they power the fridge in the van so are always connected. Early in the morning after they have been off charge for a while they read 12.66 when not under load. If the fridge comes on ~7A they will read ~12.3 to ~12.2. I have verified that the voltage at the batteries is almost exactly what the montior reads out under charging and discharging conditions. I don't know how else to test or improve the connection resistance but I would love feedback and I will go test it.

      The voltage differences between batteries is also not as dramatic when not being charged. I can pull that information as I don't have it on hand but they settle in with much less spread. I assumed the voltage difference coudl be because of the sulfation or due to connection resistance differences. If you can give me some suggestions on how to test that I will be glad to do so.

      My hydrometer only reads to the hundreth and even that is a little difficult to determine with true accuracy. It is very accurate to the 5/100th as that is its smallest deliniation. should I look at a new or different hydrometer?

      Thank you for the response and the help!


      • #4
        You're can't charge and EQ at the same time, EQ will start when charging is complete (depending on Ur charger and settings ). EQ is a form of charging that is a controlled over charge (so to speak). In most chargers it occurs at a the end of a charge cycle if u have it turned on to EQ. Some CCs have a setting to EQ in regular intervals.
        285Wx9 / MNClassic 150 / CSW4024 / TrojanL16H-ACx4


        • #5
          My controller does not have an eq setting. It's a pretty basic controller. But I can set the peak absorb voltage which I will set up to 16.2.

          If you have other suggestions on how to perform this then let me know. How do other people eq with solar? By the time you are close to fully charged you are losing sun. Withy set up they will have seen see draw by morning and will have to start over in absorb Wich eats up you insolation time.

          I asked trojan about this but they didn't seem to have much concern about having a c/20 rate while at 16.2 v

          Thank you for the help?


          • #6
            You need to start your generator in the early pre-sun morning, burn a couple gallons of fuel getting the batteries well into ABSORB, and then after they are charged by 10am, let the solar take over the long process of EQ. But your batteries need several hours of absorb FIRST, then you can EQ.
            If you don't have a generator and beefy charger, you are SOL.
            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



            • #7
              Ok, I can take a drive early morning and get the bank pretty topped off at a voltage of ~14.4v from the alternator. That should reduce the charge current as the panels take over. I expect just the jump in charge voltage from the alternator (14.4v) to the panels (16.2v) will cause the bank to resume taking some current. Maybe 10A or more which would be C/40 for this bank.

              I could likely sustain that voltage from ~9 or 10am to maybe 4 or 5pm depending on the sun. I'll have to pick a good bright day and clean the panels to give myself the best chance. Even so, from reading on here I'm not sure that is enough time?


              • #8
                Mowen you are just mow discovering why you must have a generator to go off grid. Solar is not capable of fully charging batteries and no chance to equalize. It takes up to 24 hours to properly EQ a battery. Any FLA battery must be Equalized the day they arrive, and around once a month as part of preventative maintenance schedule. In addition to maintenance a generator is also required to get you through cloudy spells, and ran once a week to top off the batteries and get them fully charged.

                Without a generator, is a death sentence for your batteries.
                MSEE, PE