Battery Charge Balancing


Trying to get an even charge (even specific gravity readings) across my battery bank. I've posted an image to show how they are currently wired together and connected to loads.

From the image, my specific gravity readings are always lower going from battery 1 to 4. Typically battery 1 will be 1.270 -1.275'ish when charged and battery 4 will be 1.240 -1.245'ish at the same time. Battery 2 and 3 readings are somewhere in between, 3 being lower than 2, and higher than 4. Roughly speaking battery 1 - 1.270, 2 - 1.260, 3 -1.250, 4 - 1.240.

I've read and re-read the article from SmartGauge about balancing, but don't see how it applies to 6 volt battery connections. I currently have all my charge controllers (3) connected to the positive and negative busbars, the positive feeding the battery bank at battery 1, leaving at the negative on battery 4.

How would I go about wiring this up so that battery 4's sg is closer to that of battery 1. I currently have about 70 amps charge current from the 3 controllers feeding the bank, with an average of 7 amps discharge for 16 hours a day, 1-2 amps through the night. Needless to say I can go many days without running the generator if I don't have sun. Is this somehow contributing to the unbalanced specific gravity readings? The bank is never below 80%, can this damage the bank if it's not getting a good prolonged charge every once in a while? The battery manufacturer (U.S. Battery) recommends equalizing every 30 days at 16 volts for 2 hours which I have done, but I still can't keep them even.

I've checked the current while charging with my clamp meter and the readings are always within .5 amps from the positive coming in, to the negative going out. Right now with the batteries basically full (97% on Trimetric) the readings were, 22.05 amps charge current on battery 1 positive coming from the busbar, 21.7 amps going out the negative from battery 4.

Thanks for any help, ideas.


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  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    Re: Battery Charge Balancing

    Bascially, you just hit a battery bank with higher voltage (~15.0 to 15.5 volts for a 12 volt bank--and make sure to turn off your expensive loads in case they don't like 15.5 volts and decide to fail) and charge until each cell reaches its maximum S.G. and there is no further movement (ideally, check every 30 minutes).

    At that point, stop the equalization, write down the numbers, and call that 100% full for each cell.

    Check that you have roughly balanced equalization current through each bank (i.e., not one bank with 10 amps and the other with 1 amp).

    Also, if this really bothers you... measure the voltage across each 6 volt battery bank... Say A and B battery have a bit "lower" equalize voltage and C and D have higher voltage.

    You can move the batteries so that A+C form one string and B+D form another string to balance out the voltage. It probably is not going to help much--but batteries are sensitive to minor voltage variations and if you can balance the total string voltages among parallel strings--it will help keep battery currents and voltage balanced.

    Other than that--Assume your measured fully equalized SG readings are it. There are some reasons why they may not match--but you cannot do anything about it now (acid fills not identical, variation in manufacturing, variation in age, variation in battery cycling and some sulfation forming, etc.).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MuskokaMuskoka Solar Expert Posts: 103 ✭✭
    Re: Battery Charge Balancing

    Hi Bill,

    Thanks for taking the time to answer.

    "At that point, stop the equalization, write down the numbers, and call that 100% full for each cell."

    This is basically what I have been doing, and the reason for my concern. When the bank is 100% charged according to the Trimetric, and not taking any more charge (the whole bank), and the controllers have gone to float mode, the sg's can vary from 1.270 battery 1 to 1.240 battery 4. The first reading is fine, it's full, but 1.240 is not full, and over time will I not be having issues with batteries 2,3 and 4?

    I have swapped the position of battery 1 and 4 to see what happens, too heavy to do that often. The bank is only 3 months old and I don't want to have things not wired optimally. When I equalize I do the whole bank at once, not individual parallel strings, is that ok, or should I be doing battery 1-2, 3-4 separately?

    I've also noticed that they don't get bubbling really good until the voltage is over 15, probably closer to 16. It is cold, and the battery room is always around the freezing mark in winter. Is it unreasonable to expect all 4 batteries to have close sg readings? I only ask because I don't know. Prior to these batteries I had a much smaller system with 4 marine 12 volt deep cycle batteries that lasted 5 years, and I never took an sg reading, so I'm new to this kind of maintenance.

  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery Charge Balancing

    Try rotating the batteries in position for a few weeks and see how they behave. Mixing up their location might be a good thing,
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Charge Balancing

    Essentially you have got it wired according to the Smartgauge "method 2". Two six Volt batteries in series become one 12 Volt, and you're connecting the (-) to one set and the (+) to the other. The bus bars are somewhat redundant.

    This may be too obvious but ... what is the wire gauge connecting the batteries, especially between batteries? There might be some high resistance in connections causing trouble too.

    Try taking one set of batteries "off line" and equalizing just one bank at a time (15 Volts for 1 hour, rest, check SG, repeat until no improvement is had). You're quite right in your assumption that the cells should be fairly equal across all and that if they aren't there eventually will be trouble.
  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    Re: Battery Charge Balancing

    There is nothing you can do practically to get the sg in balance. It is what it is.

    Equalization is very hard on a battery and you only want to do just enough to fully charge each cell. And you only want to do that, roughly, once every 2 to 3 months.

    I would only move the batteries once, if you want. Some people find that their end batteries charge differently and move them more often. Personal choice.

    You need to measure your battery temperatures too. Both specific gravity, resting, and charging voltages rise a batteries get cold. Freezing is pretty cold... Below 0F, you really need to be careful that the batteries are correctly charged and not discharged too much and risk freezing/splitting your batteries.

    If the batteries get much below freezing, you may want to think about insulating your box/bank.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,481 ✭✭✭✭
    Re: Battery Charge Balancing

    Are these batteries the same age? Check their manf. date code melt branded into the case.

    When checking with clamp on amp meter do the strap between series legs, cable between batteries 1,2 and cable between 3,4.

    This will check the charge balance between the two series strings.

    String 1,2 current + sting 3,4 current = total charger current.

    Lastly, you can disconnect the strap between good string 1,2 batteries and equalize just the 3,4 batteries.

    If all three cells in a battery have low SG reading then you can be pretty sure they are not getting enough charge or are sulfated.

    If batteries 3,4 are older, or previously used independently, they may have some aging with some hard sulfation.

    There can be manf. electrolyte mix tolerance from lot to lot of batteries. It should not be greater then 0.02 for same manf. / type battery.

    A new(er) battery may have higher SG reading as it may not have a fully formed coating of lead oxide on the positive plate.
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