What batteries for an off the grid system?

BetoBeto Registered Users Posts: 2

Hello I am looking for help to decide what batteries to buy for an off the grid system? Since this will be installed on a ranch where I might spend a month without visiting from time to time, and the batteries will be inside the house,  I am looking for a non maintenance type off battery. I was given the suggestion on buying 4 fullriver DC224-6 sealed AGM or 4 centennial CB6-224 Sealed VRLA AGM. I have seen the possibility of buying 2 fullriver DC400-6, I know it will have lower Ah, but will it make a difference to have 2 415 Ah batteries in series to having 4 224 Ah batteries (this will be connected to make 448AH)? This system will be used to operate a refrigerator, about a dozen light bulbs and from time to time a sealing fan, it was calculated to use 1,300 Whr/d. 

Thank you


  • EstragonEstragon Registered Users Posts: 3,039 ✭✭✭✭✭
    There are pros and cons to each approach.

    Having more smaller batteries has the advantage of being easier to handle, and having two strings in parallel lets you limp by on a single string if/when a battery goes bad.  Smaller batteries may also be a bit cheaper ($/watt-hour) and easier to find than larger.  On the downside, there is more wiring involved (cost and more potential trouble spots), and keeping parallel strings balanced can be an issue (especially if more than 2-3 strings).  If they were flooded type, I'd definitely lean more to fewer larger batteries.

    Having larger batteries has the advantage of less wiring, fewer individual 2v cells (lower probability of a cell going bad?), and may be more expandable if it turns out your loads are higher than expected when you start using the system (you could add a second string if needed).

    The amp-hour difference isn't really much of a factor IMHO as they're pretty close.
    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
  • BB.BB. Super Moderators, Administrators Posts: 28,184 admin
    FLA batteries need maintenance about 1x per month (check electrolyte levels, add distilled water as needed) and need recharging 1 day per month (if not left on float charge).

    AGMs, no maintenance, more expensive, a bit less life (typically) vs FLA of same quality. Can go 3-6 months between charging cycles in storage (cold batteries, lower self discharge, longer battery storage life).

    Batteries in house--Besides the issues of fire (shorted wiring, etc.). There is overcharging (failed charge controller). Always assume the worst (fire resistant materials, using UL listed hardware, fuses/breakers, conservative design).

    And AGM batteries, when they age and eventually fail--They can vent (or even catch fire)--Make sure in cool/vented space. Venting batteries can mist electrolyte (sulfuric acid+distilled water).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Marc KurthMarc Kurth Solar Expert Posts: 448 ✭✭✭✭
    edited September 21 #4
    In comparing those two brands, I would suggest comparing the manufacturers published rating data for the number deep cycles at various depth of discharge ranges.
    Remember that some manufacturers use 80% of original capacity as being the end of their lifespan. Others use 60% - quite a difference.
    All AGM batteries are not the same.

    I always have more questions than answers. That's the nature of life.
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,137 ✭✭✭✭✭
    I would add into your battery list that having any a power system in the house can bring lightning into the home. One of the first to warn about this was John Wiles who wrote alot of the safety code for solar and did alot of failure analysis. Back in the 1980's he had "ball lightning" floating around in his living room. His wife was not very happy with him I seem to remember. ;)


    Good point Marc! Also add on the chance of them being in business when you need them.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • BetoBeto Registered Users Posts: 2
    Thank you all for your comments, I really appreciate them... I will look into them
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