Battery question... One 12v or two 6v

lazykdanlazykdan Registered Users Posts: 3

Is there a rule of thumb as to which battery scenario is 'better': one 12v or two 6v in series? What are the pros/cons of each?

My use case is a small system to power a pond and a fountain.  The solar panel will be on top of a nearby storage shed and all the electrical components will be inside the shed. 

Thanks in advance!
Dan

Comments

  • littleharbor2littleharbor2 Solar Expert Posts: 704 ✭✭✭✭
    One typical 12volt battery will be about 100 - 120 amp hours One typical 6 volt GC2 golf cart battery will be 210-240 amp hours. If you need 100 - 120 amp hours then the 12 volt battery is fine. If you need more  you shouldn't parallel 2 12 volt batteries when you can series 2 6 volt batteries. Many 12 volt batteries like the "deep cycle RV/marine" batteries aren't true deep cycle batteries. GC2, golf cart batteries or something like a Trojan T-105 are deep cycle batteries and will give you better daily cycling performance. 

    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.

  • lazykdanlazykdan Registered Users Posts: 3
    Thanks for the info. I do need about 100 ah and was initially considering one 12v but this has made me reconsider.  Having 200+ah would allow me to throw some lighting onto the system as well and would be a good safety for even more cloudy days...not that we have too many of those here in PHX.
  • EstragonEstragon Registered Users Posts: 1,616 ✭✭✭✭
    Just to clarify, you don't generally want to draw down a deep cycle lead acid battery below 50% state of charge for decent battery lifetime, so a pair of golf carts would give you ~100ah of usable capacity.
    Off-grid.  
    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
  • lazykdanlazykdan Registered Users Posts: 3
    Understood...to clarify, if my math is correct, I actually only *need* 45ah which is how I got my 100ah sizing.  That said, 200 is still better than 100... 

    Thanks for the assistance!
  • tabbycattabbycat Solar Expert Posts: 41 ✭✭
    When you double your battery capacity you need to double your absorb charging current. If you look at battery manufacturers websites they recommend a finish charge of three percent of battery capacity.  For a 100 AH bank that is three amps; for a 200 AH bank that is six amps. Is your panel capacity up to this requirement?
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