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Storage of batteries below grade to stabilize temperature geothermally

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  • Storage of batteries below grade to stabilize temperature geothermally

    Newbie here. Looks like a good place to ask questions. I will be posting at least one other question about an off-grid system to power my home, but this question I believe fits well here.

    I am planning to install a PV system with generator backup on/in an unheated detached garage, and run the AC output 40' to my house. My idea was to custom slope the roof (about 36 degrees from vertical) for the fixed array, and everything would be located in this new building. I'll be using about 7kWh a day. This will be located in the lower Adirondacks, near Forestport NY.

    I am wondering if it makes sense to position the batteries just below grade, to take advantage the insulating properties of the ground. I haven't chosen the kind of battery yet, and honestly I was looking at the Aquion AHI which based on the posts here looks more doubtful than I was hoping.

    The AHI is a sealed battery, and would be lower maintenance than a lead acid battery. Wouldn't outgas either. But I digress - that is a whole other ball of wax (or vapor). Setting aside the kind of battery for just a moment, and the fact that the kind may determine the need for better/more frequent access/venting etc, does it make any sense to try to keep the batteries at a certain temperature or a range to maximize life and/or performance? Am I dreaming that putting them below grade will prevent them from going below 40 degrees? I was even dreaming about putting a low wattage heater (yes, I know this is off-grid solar ) in a confined storage space, but I can imagine with certain kinds this may not be advised.

    I can't be the first person who has wondered about this. Do most people put the batteries in the house (basement), so this isn't an issue? My main driving force is I can't place the panels on the roof of the house, which faces East. By building, I can get due South. But now I am in a detached building that isn't a living space and will be unheated. This question could be a lot larger than I thought, but the reader can see my situation. I suppose I could use a small propane heater to heat an equipment room, thought I don't know of safety concerns. I'm not sure a Generac is going to like a totally unheated area either, and I am not sure how to keep that toasty since it needs to exhaust. It will be needed to be available each day, as the insolation may be a shade under 2.0.

    Thanks in advance,
    Don

  • #2
    Batteries, when charging or discharging, generate some heat. If you can insulate them in a nice cozy box (that can let the hydrogen vent away) that is often good enough. Be sure that in summer, the heat can escape.

    Going underground is a pretty radical step, harder to get down to them to check fluids and such.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      Originally posted by Mike90250 View Post
      Batteries, when charging or discharging, generate some heat. If you can insulate them in a nice cozy box (that can let the hydrogen vent away) that is often good enough. Be sure that in summer, the heat can escape.

      Going underground is a pretty radical step, harder to get down to them to check fluids and such.
      That's a great point - in warmer weather, if the batteries are enclosed, they will be too warm. I agree that based on kind of battery used, the needed maintenance will factor into the design. Until coming to this forum, I was assuming I was using the AHI, which I don't believe require regular maintenance. My thinking is, I'm building from scratch, why not build around the solar system as much as possible. I am now discovering that while worrying over all the circuit design details, I may have missed the fact that an unconditioned building may present some issues.

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      • #4
        Batteries capacity are specified at 77 degrees. End of story.

        However that does not mean they have to be kept at 77 degrees. What is important for you to understand is how temp effects the batteries. Heat is a batteries worse enemy or above 77 degrees. When warm a battery capacity increases, but voltages must be lowered. Cold batteries last longer, but have reduced capacity and require higher charge voltages.

        Takeaway is determine at what temperature your batteries will operate in (coldest) and then derate them to proper capacity. In addition use temperature compensation chargers.
        MSEE, PE

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        • #5
          Originally posted by Sunking View Post
          Batteries capacity are specified at 77 degrees. End of story.

          However that does not mean they have to be kept at 77 degrees. What is important for you to understand is how temp effects the batteries. Heat is a batteries worse enemy or above 77 degrees. When warm a battery capacity increases, but voltages must be lowered. Cold batteries last longer, but have reduced capacity and require higher charge voltages.

          Takeaway is determine at what temperature your batteries will operate in (coldest) and then derate them to proper capacity. In addition use temperature compensation chargers.
          Thanks. I have a lot of silly questions, because I am new. I spoke with an installer who has lived off grid many years today, and he gave me lots of advice. He said I was on the right track in thinking of insulating the batteries to some degree from wide temperature swings to obtain the greatest capacity. My concern came from all the equipment being stored in an unheated/cooled building. What I came to realize and he confirmed is if I go with higher voltage/lower amperage DC from the array on the roof of the outbuilding, I can put everything else in the basement of the house - as long as I plan for what to do about batteries that may present a hazard. He has used methods of venting that include both fresh air intake and exhaust. He also pointed out what can be done with heating mats, small propane heaters carefully installed, etc to heat the equipment area if storage is done in an outside building. It was as I was using the Schneider Conext string software late last night, which includes an analysis of the losses in wiring for different sizes, that I got the bright idea to move all the stuff except arrays into the house, since I only have 18 amps or so DC.

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