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mixing slightly used batteries with new

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  • mixing slightly used batteries with new

    My current 24V off-grid system has 4 Trojan L16 in series. The batteries have been operating for 8 months at a daily DOD of 20%.
    I want to upgrade to a 48V system and add 4 new L16's so I would have total 8 batteries in series. Would this mix of battery ages reduce the longterm capacity of the battery bank substantially?

  • #2
    Originally posted by Gyro View Post
    My current 24V off-grid system has 4 Trojan L16 in series. The batteries have been operating for 8 months at a daily DOD of 20%.
    I want to upgrade to a 48V system and add 4 new L16's so I would have total 8 batteries in series. Would this mix of battery ages reduce the longterm capacity of the battery bank substantially?
    Probably not, but keep an eye on SG of each set of batteries and you may have to do more frequent EQ or at least long Absorb.

    When the bank eventually dies, replace all 8 batteries at once even if only four of them are bad.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      While adding new batteries to old ones is a bad thing at least by putting the new ones in series will probably just shorten the life of the new ones somewhat.

      As inetdog suggests, keep an eye on the SG and next time consider all 8 new batteries when this bank starts to fail even if it is only one of the 8 that is going bad.

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      • #4
        You will be fine. Your biggest issue getting them all EQ'ed. In other words you have to get every battery fully charged up first. You have to start at a known reference point of either at the Top or Bottom. With FLA you can only reference at the top. If it were me I would put them all in Parallel and an EQ charge for 24 hours.

        For you, I would configure the new set for 24 volts, and use your AC charger to get them fully charged up, and then add your old set in parallel and give them both a good 24 hour EQ charge. Then assemble in 48 volts ready to go. Takeaway, get all batteries fully charged before going live.
        MSEE, PE

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        • #5
          Excellent! Thank you all!

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