Storage SLA's with slightly different voltages

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  • St8kout
    Junior Member
    • Feb 2016
    • 7
    #1

    Storage SLA's with slightly different voltages

    I've got four 12v SLA 155ah storage batteries made for renewable energy. All were bought from the same company but 4 months apart, (two in Dec and two more recently). None were used yet.

    After topping off each one separately with a "Battery Minder Plus" unit, there are some voltage differences between the 'new' and 'old' pairs after giving them a few days to settle down after charging. One is 13.17v, another is 13.11v, while the two 'new' ones are 13.07v. Can I assume this is not enough voltage difference to matter for a series-parallel setup for a 24v Inverter, as long as I mix and match them for the lowest voltage difference between parallel strings, (one 'old' and one 'new' in each series string)?

    I understand that the internal resistance of these is really low, so a voltage difference between parallel strings can lead to a rapid high-current dump from the high string to the low string. Any idea what would be the highest voltage difference allowed between strings?

    BTW, I will be using the 4-battery series-parallel wiring diagram posted elsewhere here that shows the optimum place to connect to the charge controller and Inverter, so I got that part covered.
    Tags: None
    • Mike90250
      Moderator
      • May 2009
      • 16020
      #2
      I would suggest paralleling the LIKE voltage batteries.
      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

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        Comment

        • PNjunction
          Solar Fanatic
          • Jul 2012
          • 2179
          #3
          Strive for no more than 0.1v difference among the batteries.

          I commend you for charging and topping each one off and keeping an eye on it prior to major service.

          Once in service, expect the battery voltages to vary a little bit for the first 10 cycles or so as they get *additional* plate formation from normal use. They should settle somewhat after 10 cycles. If you want to be very cautious, take the time to top them off again with the minder after those 10 cycles. (not for normal deep cycle charging!).

          Keep looking for any battery more than 0.1V out from the other and reapply the minder to *all again* if necessary. If the batteries aren't TOO different in age and capacity, the interval between the time that the terminal voltages try to exceed 0.1v will get longer in duration.

          IF at any time you have an accidental heavy deep discharge, and bulk charge back up quickly enough in time so as not to damage them from hardened sulfation, be SURE to put the minder back on each battery for an individual top-off after the major bulk charge is done.

            Comment

            • SunEagle
              Super Moderator
              • Oct 2012
              • 15151
              #4
              I would also recommend breaking down the system and checking each battery SOC separately after a a couple of months of usage. That will tell you if any are being abused or is the weakest.

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