batteries in parallel or series parallel?

quartersplashquartersplash Registered Users Posts: 1
I am new to the forum. 
I have looked high and low online for videos or a diagram of how to connect 8  12 V batteries in a 24 V system for my solar system. I am trying to find out if I should connect them all in parallel or in serious parallel and also a diagram of how to do that. I have a 3000 W Victron inverter charger and a 250 volt Victron charge controller.  If someone can help me with that I would greatly appreciate it. Thanks forum. 
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  • 706jim706jim Solar Expert Posts: 472 ✭✭✭✭
    Series parallel
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 29th year.
  • BB.BB. Super Moderators, Administrators Posts: 32,593 admin
    Welcome to the forum QuarterSplash,

    Can you tell us a bit more about your needs and/or system requirements & design?

    Details matter here... For typical batteries (Flooded Cell Lead Acid, AGM, GEL, etc.), the batteries are typically connected in series first to set battery bus voltage, then connected in parallel to set the bank Amp*Hour capacity.

    With Lithium Ion batteries... Typically the cells are connected in parallel, then connected in series. The result is the same (xx Bank voltage, yyy bank AH capacity). However, this is done because Li Ion batteries generally have a Battery Management System (BMS) that is connected to each group of cells to measure their voltage. If the cell voltage goes too low or too high, the BMS will set an alarm or possibly disconnect the battery bank to prevent ruining the cells/bank.

    Also, with batteries... They come in different voltages. For lead acid, typically 2/4/6/8/12 volts (1 cell to 6 cells per "battery" or group of cells). And there are a wide range of cell AH capacity.

    For lead acid batteries, I suggest the "optimum" battery bank typically consists of 1 to 3 parallel strings. Having a "whole bunch" of lead acid batteries in parallel (seen upwards of 12x smaller 12 volt batteries in parallel) is something that is better avoided (wiring issues, maintenance of connects and cell electrolyte levels, etc.).

    For example, say you are choosing 8x total batteries that are 12 volt @ 100 AH per battery. For a 24 volt bank:
    • 2x 12 volt batteries in series = 24 volts
    • 4x 100 AH battery strings in parallel = 400 AH
    • 8x total batteries x 6 cells per battery = 48 cells to check
    But, you could also choose to use 8x 6 volt @ 200 AH "golf cart" batteries. Physically the same size and weight as the 12 volt @ 100 AH battery string.
    • 4x 6 volt batteries in series = 24 volts
    • 2x 200 AH battery strings in parallel = 400 AH
    • 8x total batteries * 3 cells per battery = 24 cells to check
    There are even high capacity 2 volt cells... Say 2 volts @ 400 AH
    • 12x 2 volt @ 400 AH cells in series = 24 volt @ 400 AH battery bank
    • 12x cells * 1 cell per "battery" = 12 cells to check
    Personally, I recommend the second configuration (6 volt @ 200 AH batteries) for various reasons (such as 6 volt @ 200 AH "golf cart" batteries are cheap and available)... The third with 2 volt batteries is a good solution too--But they tend to be more expensive than a "Golf Cart" bank.

    Of course, this also depends on what the actual capacity of your batteries are, and what batteries are available in your area (and if you are using flooded cell, or AGM, or even Li Ion batteries with internal BMS, etc.).

    There are lots of ways of doing this... If you can tell us more about your bank, we can better help you. 

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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