Extended time away from off grid cabin

donwdonw Registered Users Posts: 4

mornin - new to the forum and I hope the experts out there could lend some direction.

Small offgrid solar / battery set up for cabin, will use 6-8 months out of the season. At long periods of non-use time should the system be run thru a diversion load similar to what a wind generator might use ??

Thank you in advance for your help



  • BB.BB. Super Moderators, Administrators Posts: 30,058 admin

    Welcome to the forum Don,

    If you have flooded cell batteries--Make sure that the controller is working reliably, turn off and other loads. And you want the controller to only "float" your battery bank (something like 13.6 volts charging for FLA battery).

    The problem with flooded cell batteries is that they need continuous float charging or regular charging 1 day a month to prevent/reduce sulfation (have relatively high self discharge). And, you need to check the water levels every month (ideally) to make sure the plates are never exposed to air. Keeping the float voltage low, should reduce water usage greatly.

    If you have AGM batteries, they can be stored disconnected (no charging/discharging) for up to 6 months between chargings. They have relatively low self discharge.

    Also, the temperature of the battery bank matters too... Battery Specs are at 75F/25C. For every 10C(18F) reduction in temperature, batteries will age 1/2 as fast, and self discharge will be ~1/2 as much too (i.e., at 57F, an FLA battery could go ~2 months between charging).

    Conversely, if the battery bank is in a hot climate, for every 10C/18F increase in temperature, battery bank will age 2x faster and have 2x higher self discharge.

    The other option--Bring the battery and any "expensive" back home for security (remote cabins, things can "grow legs").

    Use a "float charger" on the battery bank:


    I have a few BatteryMinder brand float chargers on cars and trucks that are not driven much--And they have been great. Keep the batteries charged, and the batteries do not gas/use water (other "cheap" float chargers, still try to "boil" the batteries dry after 6 months).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • donwdonw Registered Users Posts: 4

    Thanx BB - I appreciate the input, so with the low discharge of the AGM batteries would they also be a better choice for colder climates i.e below 32 deg ? It wouldn’t be impossible to bring batteries out of the woods but it would be very inconvenient and ya I know sometimes things grow legs ……

  • BB.BB. Super Moderators, Administrators Posts: 30,058 admin

    AGM batteries are more "freeze" resistant (are not supposed to crack the case and leak) regardless of state of charge.

    Flooded Cell Lead Acid batteries are very freeze resistant when fully charged, and will freeze just below 32F when "dead" (the usual warning, taking Lead Acid Batteries to "dead" or 10.5 volts, they are usually junk after this):


    Freezing a lead acid battery leads to permanent damage. Always keep the batteries fully charged because in the discharged state the electrolyte becomes more water-like and freezes earlier than when fully charged. According to BCI, a specific gravity of 1.15 has a freezing temperature of –15°C (5°F). This compares to –55°C (–67°F) for a specific gravity of 1.265 with a fully charged starter battery. Flooded lead acid batteries tend to crack the case and cause leakage if frozen; sealed lead acid packs lose potency and only deliver a few cycles before they fade and need replacement.

    And FLA batteries should be >75% state of charge when stored (charge to 100%, then 1 month self discharge to 75% SoC).

    Once you warm up the cabin (and battery bank) to >40F or so... Then either FLA or AGM will be fine (all batteries lose capacity when sub freezing). FLA will tend to keep warm when used/charged/discharged from internal resistance and gassing (which also generates heat). AGMs are much more efficient, so you might need to add a battery heater to the battery(ies) to get them warm.

    The price difference between a pair of 6 volt @ 200 AH "golf cart" batteries at ~$80-$100 each... Vs the same battery in AGM:


    At ~$335 each... You could "replace" your golf cart FLA batteries every 2 years and still probably save money. But then you have the hassles of replacing the batteries every two years (and packing the old ones out), and bringing distilled water to keep them full (some folks use rain water off a clean roof for battery water--your mileage may vary).

    Cheap golf cart batteries may last 3-5 years (at room temperature). Good Quality AGM may last 5-7 years. And any battery misused/abused can be junk by tomorrow. 😥


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • donwdonw Registered Users Posts: 4

    ok thank you once again - I’ve been eying the 6v Trojan T145 wired series / parallel bank of 6 with 6k watt inverter to start with.

  • BB.BB. Super Moderators, Administrators Posts: 30,058 admin

    Personally, my preference would not be 2x 6 volt in series by 6 parallel strings (6v @ 200 AH x 2s x2p = 12 volts @ 1,200 AH).

    I would be suggesting 4x 6 volt @ 200 AH in series by 3 parallel strings (24 volts @ 600 AH).

    Both configurations store the "same amount of energy". Generally a higher voltage/lower current battery bus is easier and cheaper to make.

    If you can avoid >3 parallel strings of batteries, it is usually easier to maintain and cheaper to wire... And (solar chargers especially) chargers and equipment draws/supplies less current. Smaller wire and longer runs supported (1/4 the amount of copper and 1/4 the cost of copper).

    For example most solar charge controllers will work at 12 or 24 (or even 48) volt battery bus. But a 60 Amp Charge controller is limited to 60 Amps. However, the solar array supported at 24 volts is 2x larger than the 12 volt array support. For an MPPT type charge controller:

    • 60 amp charge controller * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,230 Watt array @ 12 volt
    • 60 amp charge controller * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,460 Watt array @ 24 volts

    Your battery bank should be charged at 5% to 13% rate of charge, and if more than a weekend/seasonal cabin, then 10%+ recommended rate of charge:

    • 1,200 AH (at 12 volts) * 0.10 rate of charge = 120 Amps nominal rate of charge
    • 600 AH (at 24 volts) * 0.10 rate of charge = 60 Amps nominal rate of charge

    When wiring up your parallel string, take a look at this website:



    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • donwdonw Registered Users Posts: 4

    the last of the four is what I was studying /


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