Weekend cabin extreme makeover

We have a small cabin, 14x26, that is totally off-grid. For the past couple years, I've been running a small fluorescent 12v light off my deep cycle boat batteries, as most of the use at the cabin has been in the fall after boating season. We recently did a remodel, and I bought a TV to for the cabin, and also found a 2500W inverter on craigslist that I have wired into the breaker panel . The cabin was moved to the current location from a location where there was power, and we have run it off a generator in the past. I don't like the noise and the gas smell of a generator, as we do a lot of hunting out of this cabin and I believe the noise has an affect on game, and I know that the smell is not good for hunting.

Besides being somewhat seasonal use, the cabin is mostly weekend use as well, other than one 5 day, and one week long outing, both in November. We are in northern Wisconsin, at ~45.6 degrees latitude, so we're looking at about 2.5 to 3 peak hours of solar when our usage is the highest.

I recently was offered four sealed 170aH 12vdc lead acid batteries from a telco replacement, and I'd like to be able to up the use of power at the cabin... a 12vdc cooler maybe, a crockpot every now and then,, maybe an electric toaster, and for my bride, when she joins me there, a blow dryer. I think for the most part we can keep it to under 2kw/day, but as I see from being a troll, that will likely grow.

I have 680aH of battery, that are made to float and provide telco backup of 80aH for 8 hours. While this is overkill for my usage, how could I turn down the offer of these batteries?

I want to go solar, and I think I will probably build my own panels, I'm cheap and somewhat capable. What do I need for a solar build to keep a maximize the longevity of the batteries? At the C/20 if I calculate this correctly I need about 400W. Can I go below C/20, since my usage will be low and not consistent? I think i could probably get by with half the battery plant that was given to me, but then, what do I do with the other two? Can I find a way to keep them that will be good for them without using them?

Thanks, this place is great, I am learning a lot, but there is so much more!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Weekend cabin extreme makeover

    In general, any large electric appliances (hair dryers, crock pots, possibly even a 12 volt cooler) is going to be hard to justify the costs for in a low solar energy location (winter hunting or summer shade?).

    You have several ways of approaching the problem... One is based on the hardware you have (battery bank) or based on how much power you will want/need.

    Lets just start with how much power you will need. For a off-grid home, 3.3 kWH or 100 kWH per month is a reasonable point to plan for. Significant power but not too large of system. Lets say 2.5 hours per day, 0.52 system efficiency (from solar panels to 120 VAC power), fixed array (also do your 2kWH per day):
    • 3,300 WH per day * 1/0.52 * 1/2.5 hours sun per day = 2,538 watts of solar panels
    • 2,000 WH per day * 1/0.52 * 1/2.5 hours sun per day = 1,538 watts of solar panels
    Not a small system in either case...

    Battery bank size... Typically, you can run 1-3 days on your battery bank to 50% maximum discharge (for long life). For a lightly used cabin/RV, you could go to 80% discharge and the batteries will still probably age out rather than wear out (at least 200-500 cycles?). Check the battery manuals for cycle life information.

    Lets say you run your batteries on a 50% discharge for 1 day of no sun. Also the high side of 3.3 kWH and 3 days of no sun:
    • 2,000 WH * 1/0.5 max discharge * 1 day * 1/12 volts = 333 AH @ 12 volts
    • 3,300 WH * 1/0.5 * 3 day * 1/12 volts = 1,650 AH @ 12 volts
    So, that gives you a practical range for your battery bank. Note, for the larger battery bank you may want to look at 24 or 48 bank voltage unless your loads remain fairly low (1,200 watts or so maximum for 12 volt system is pretty reasonable).

    I will stop here--Please feel free to ask question and modify my assumptions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Weekend cabin extreme makeover

    Thanks for the reply Bill.

    The crock-pot I'm considering is a small one... 150W, would try to keep it down to 4 hours max. The 12V cooler I'm looking at is 60W as well, and probably won't use it on long term stays, since the great outdoors is the best cooler in Wisconsin in November as a rule. The hair dryer would be more than likely an anomaly, and again, not in November on the multi-day use; most of the time then it's guys at the cabin and a lot of us don't even have hair anymore :-)

    I think I can estimate a max of 2kWH/day, most days would be 1kWH or less.

    So, for most weeks, at most use would be 4kwh in 2 days with 5 days of no use and charge only. There will be several weeks of no use at all, and then, in the fall, there will be weekend use almost every weekend from early September until late November.

    Typically, there will be 5 days in early November where we'll max it out at 2kWH/day, so a total of 10kWH with only 2.5 to 3 peak sun hours per day. Thinking out loud here, that leaves 7 days of charge for 5 days of use, so figure 17.5 hours of peak sun for 10kWH of use. Then, later in November, there will be 9 days that I think I can estimate at 1kWH/day max. The batteries say they'll do 20.9aH for 8 hours each.

    I guess the big question is, with my limited use of the cabin, and since these batteries are meant for stand-by, can I plan to trickle charge them year round at less than C/20, taxing them on weekends and charging them during the week, and just beat them up a bit every fall in November on 2 weeks with a charge week in between?

    I would like to keep my solar build to 300W or less, guess I need to do some math, but my real concern is how to best preserve the longevity of my batteries (and not spend much).

    Thanks again!
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Weekend cabin extreme makeover

    I think your real concern might be how to preserve the longevity of home made solar panels. Personally, I wouldn't trust them for weeks or months at a time unattended.

    They won't be properly vacuum sealed, and so moisture, likely in the form of condensation, is pretty much guaranteed to show up inside.

    They short out, the cabin burns down and a few days or weeks later you find out about it.
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Weekend cabin extreme makeover
    bluehills wrote: »
    can I plan to trickle charge them year round at less than C/20, taxing them on weekends and charging them during the week, and just beat them up a bit every fall in November on 2 weeks with a charge week in between?

    That is the general reciepe for "Decifict charging sulfation" which will really degrade them each week you "beat them up". After 24 hours, the sulfite crystals harden and set. This locks up some capacity in the crystals. Next week, your 1,000 AH battery is only 900AH, and it only gets worse, till you are down to 50 AH, and your panels can recharge it in 1 day.
    Powerfab top of pole PV mount | 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 ,

  • mikeomikeo Solar Expert Posts: 386 ✭✭✭
    Re: Weekend cabin extreme makeover
    Next week, your 1000AH battery is only 9000AH,
    Please tell me how to accomplish this feat:)
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Weekend cabin extreme makeover
    mikeo wrote: »
    Please tell me how to accomplish this feat:)

    Typoes !
    (text padding to get the post to post)
    Powerfab top of pole PV mount | 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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