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solar panel/battery bank balancing questions

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  • solar panel/battery bank balancing questions

    Hi...New member here.
    I am pretty much self taught in this area so i am open to opinions and lessons learned. I recently purchased a small off grid cabin that was wired for AC and runs with a 3000W generator. Not a fan of background noise and lugging gas cans so i decided that i would supplement my set-up with some solar. The cabin has 6-7 led lights, coffee maker, computer/monitor and a shallow well pump. Fridge and stove are propane as is the hot water heater. So my challenge is the well pump (jet) so i found an old piston pump and reduced the motor to 1/6hp that draws about 240W. I have not yet purchased panels as we are in winter here in Canada and the cabin is already unaccessible. I started to source the rest of the materials needed and got a 40amp charge controller (12/24V), 3000W inverter/charger (12V). I found 2 sets of four 6v (230Ah) deep cycle batteries wired in series/parallel that deliver 12V. So it look like i have locked myself into 12V. According to my calculations i have 920Ah of battery capacity so i should be ok to take care of my needs including the well pump. Since i only plan on using the cabin on weekends I was told that I would only need about 1000-1200W of solar panels. In my reading...I am wondering if I may have too much battery capacity for my planned solar to deliver and keep my batteries healthy? Should i add more panels? Perhaps the generator can help equalize batteries occasionally and avoid sulfation? Any advice would be appreciated?

  • #2
    You have a few issues with your equipment.

    With 1000 - 1200watts of panels you are going to need more than a 40amp CC for a 12v system. Simple math would be to divide the panel wattage by the battery voltage. ie. 1000w / 12v = 83 amps.

    Next with a 3000 watt inverter connected to a 12V battery the wires can see 250 amps (3000w / 12v = 250a) which would require very big wires and fuses.

    Next with 8 x 6V batteries wired in a 2S4P formation you run the risk of unequal charging and discharging them due to so many being paralleled wired.

    You also need about 92 amps of charging for that 920Ah battery system.

    Finally except for that pump you really do not know how many watt hours you will burn. That is something you need to pin down to make sure your battery system is adequate.

    Oh. Welcome to Solar Panel Talk
    Last edited by SunEagle; 12-11-2019, 07:50 PM.


    • #3
      thanks for the response. The charge controller limitations was a concern. I think adding another module would not be a big issue. I am running 1Awg wires between batteries and inverter/charger to busbar with 300A fuses so I think i am ok there? As for the 92 amps needed is that possible with 1Kw of solar panels assuming about 5-6 hours of sunlight? Also remember that i only use the cabin a couple of days per week. The total usage during day is small except for pump. I estimate about 50w x 3hrs of lights. 100w x 3hrs of computer/monitor/phone charging and I can make coffee on the propane! My pump consumes 240w so if it works 30 minutes per day then i have a total of about 600w of consumption daily. Most of the solar should therefore go to charging. I may have over capacity battery wise.


      • #4
        The wire gauge tables indicate 1 ga wire needs to be fused at 120A, not 300A

        "just a couple days a week" indicates your batteries will slowly die from sulfation, just 5% loss a week. Maybe after a year, the degradation will stop (when the batteries are at 2% of capacity)

        . So you will have to run the generator on the last day there, to bulk recharge the batteries, so the solar has some chance of topping them off before you return.
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A