to string or not to string

dongringodongringo Posts: 5Registered Users
I have 6 x 105A 12 volt batteries in parallel, less than 6 months old.
Now I want to add 2 more.
Should I create 2 strings of 4 each?


  • EstragonEstragon Posts: 2,764Registered Users ✭✭✭✭
    If you have 6x12v 105a batteries in parallel, you have about a 600ah 12v battery bank.  Even at 6mos old, it may be having balance issues.

    What do you mean by "2 strings"?  That sort of implies 2 serially connected strings in parallel ([email protected]).  A better configuration, but it depends on what you're trying to do.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • TerryTerry Posts: 1Registered Users
    I hope this is helpful.
    Parallel increases ah while maintaining voltage.
    Strings (series) increases voltage while maintaining amps.
    Either method results in the same power.
    Higher voltage from the batteries is more efficient for inverters to convert to 120v AC and reduces "line loss" over longer wire runs.
    Inverters must match the battery voltage selected. (changing voltage may require a new inverter).
    MPPT Charge Controllers can convert higher Module voltage down to selected battery voltage.
    The Solar array voltage should always be higher than the selected battery voltage. (See MPPT Controller instructions).
    A very basic rule of thumb is: The Solar array nominal wattage rating should be at least 1.5 times the battery bank ah rating (at the c20 rate) for proper charging. (Research proper deep cycle battery charging for more detailed info on proper rates of charge)
    Example: For a 210ah battery bank: 210ah x 1.5 = 315w array. A 420ah battery bank: 420ah x 1.50 = 630w array.
    If the array is too small, or if it is wired at too low a voltage, your batteries will die slowly from undercharging.
    My 12v battery backup system is: 840w array wired in two strings effectively achieving 82.88v in below zero weather.
    I keep it at 12v nominal so I can add in my Camper batts in the winter. (24v would be more efficient and require a new inverter.)
    4.8kw Grid Tied Array
    840w Battery Backup array
    340w camper array
  • littleharbor2littleharbor2 Posts: 1,009Solar Expert ✭✭✭✭
    Terry said:

    Example: For a 210ah battery bank: 210ah x 1.5 = 315w array. A 420ah battery bank: 420ah x 1.50 = 630w array.

    That method won't work too well on a 210 ah. @ 48 volt bank. 315 watts wont work so well. Are you referring to 12 volt only?

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

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