my 12v offgrid system

Discussion in 'Energy' started by stevo, Jul 6, 2013.

  1. stevo

    stevo Backyard Farmer Premium Member

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    That's some interesting information there Scott. It's interesting to see what others are doing. As you've commented on, electric heatig devices like stoves or water heaters use heaps of power, i found a small electic camp oven thing online (i didn't buy it) and worked out that i could run it for about 2 hours with my battery capacity but then i have nothing left, it doesn't concern me, i find it pretty funny working these things out with my small system.

    Question for you Scott, this might sound silly, for battery longevity is it better to:

    1. Have a larger solar power capactity than what you need and keep the battery voltage high with less changes (13v - 14v)
    or
    2. Have a power solar panel capacity that just provides enough power to top up the batteries and you end up running the batteries down to 12v more often?

    Is it called something like cycling and float? I've read a fair bit of stuff but i don't take in everything.
     
  2. Scott Mac

    Scott Mac Well-Known Member Premium Member

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    Hey Stevo,

    Your asking a big question there with a lot of explaining needed by me. Bear with me as I ramble on and thry make it as clear as I can with out the tech terms which most guys love to use to show off their knowledge.

    Battery cycle for long life of a battery bank. Example figures are not exact, just to explain the characteristics.
    Best performance and life from a battery bank is to calculate how much power you use a day in amp hours and try to have a battery bank 5-10 times the amount.
    ie - we use 160 AH, amp hours a day on a 24V system. Our battery bank is a 24V 1600AH bank. You can go for 3 days of rain, batteries are cycled down to 60-70% and then we charge to full if the 4rth day is rain again. This is a recommended ideal for a system, However, as you can imagine, can add up in cost in batteries and panels.
    To be honest, the above sytem mentioned is mine, that runs an electric fridge and chest freezer that could cost $25-30K. You can design a system for $5-10K that will do for just lighting, tv, stereo, computers very efficiently. Gas or wood fuel for cooking and refrigeration, solar hot water boost with elec generator.

    Battery behaviour.
    regulation of batteries percentage is easier regulated by charging batteries to full with generator, next few days let the batteries be charged by the sun and check the history of AH's each day and if the bank floated or not.

    If not reached float, and you've had good sun, your not collecting enough power for your daily use.

    If floated for 2-3 hrs, then your doing well. If you collect 160 AH each day and the batteries reached float, that will give you the best idea of how many days with out having to worry about the batteries charge.

    The most accurated display of battery state of charge i'll explain latter. Kids calling.

    Hope that helps a bit

    Batteries that are deep cycled every day won't last long, maybe a couple of years, 10% cycle will give you best result with a life span around 12 years

    regards
    Scott
     
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  3. Scott Mac

    Scott Mac Well-Known Member Premium Member

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    I've got another 5 minutes

    Batteries are charged rapidly with high current and voltage when lowish. This is called bulk charge. As the the batteries get fuller, the current slows down and voltage drops. this is called absorbtion, the when full hit float with trickle type current and low static voltage.

    Another thing to remember, the batteries get a lot of power into them when low, and slow down the rate of charge current the fuller they get.
    The reason to remember this is a 60amp charger may charge full current at beginning, however slows down as it goes through the cycle. Hence if your batteries are 120ah down, it may take 3-4 hours to charge with a 60 A charger.
     
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  4. Scott Mac

    Scott Mac Well-Known Member Premium Member

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    You can add panels, but you shouldn't add new batteries to old. Batteries are best wired in series. Panels in parallel, 12v made up of say 3 x 4v 800ah batteries are much better than 4 x 200ah truck deep cycle. There is a technical reason if you really want to know, but that's the way it works. Panels in parallel in times of shade, if no chance of shade on a panel compared to others not, not as important.

    Tracking frames for panels to follow sun, expensive and moving parts to need repair or that can fail. Just add a few more panels to compensate. Same idea with stands to get optimal angle. Add a couple of panels and stick em flat to a 24.5 deg pitched roof.

    I'm off again
     
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  5. Scott Mac

    Scott Mac Well-Known Member Premium Member

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    you can, if you find you need more storage power, ie batteries, add a second bank to switch between for charging and use that are of different ages and are electrically isolated from each other. The charging can be done automatically with a dual charger type, and one bank can be solely for running the fridge and the other for the rest of the house, ie lights, gpo's to run tv, stereo, computers etc,

    before I get any further, out of Stevo's 2 options, option 1 is the best idea. I hope the above info helps to arrive to an understanding of why. If not fire me any questions

    I am often stoped in the street and asked by concerned citizens how to save energy, or about solar options. I find myself easily getting caught up for half an hour giving free advise where I could be at a job making money. If I write this all down on your site, i'll guide people to it so I can reduce my time spent in the street, and as you can see, it' would be pretty hard to retain all of this with out it being written down.

    I'm not after a job to install solar on anyone's house. I choose not to trade in this field due to people often running a system into the ground by adding too many appliances that were not in the design or not caring for their batteries well enough. The installer usually gets the blame. Plus you don't need to be a qualified tradesman to install, you can do a short course then claim to be an expert. Large companies can afford to buy panels in bulk where I can't.

    the advise or opinion of mine is totally free and a description of my conclusions from working in the past in high end Marine electrical installing and designing all types of electrical systems and power storage/charge systems, anything with a voltage on a boat.
     
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  6. armysnail

    armysnail Active Member Premium Member

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    I've just spent about a week looking at every (slight exaggeration) aquaculture video out there. I have a small system I am testing on strawberries using goldfish. After a month it seems fine. I am waiting for lettuce seedlings to grow to try them too. What's your guestimate on a starting system to run a freezer and a fridge Stevo? (ps. love your blog)
     
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  7. stevo

    stevo Backyard Farmer Premium Member

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    Thanks for all the info Scott! I have been reorganising my systems, changing regulators and adding some new panels.

    Chris, new projects! sounds good! Do you mean 12v fridge or 240v? All my stuff is 12v so i do everything with minimal budget. I have a 12v fridge, and have run it with one 85watt panel and one 120AH battery for the last two years. The battery would only run low if there was two or three days cloud.

    But, i have now put another 85watt panel on to that system, and put one 200watt on the other system for the tv, laptop, lights, fan, small stereo
     
  8. armysnail

    armysnail Active Member Premium Member

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    I was looking at 240v as I didn't even think about twelve volt. Looks like it is back to the web to look up suppliers of 12v equipment.
     
  9. stevo

    stevo Backyard Farmer Premium Member

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    12v gear might not suit if you want to run all your normal gear. if you only want a small system then it's ok. I take my fridge and panel when I go camping. most 12v appliances are expensive compared to 240v.
     
  10. stevo

    stevo Backyard Farmer Premium Member

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    I'd say if you only wanted to run a 240v fridge you'd have to spend a bit on the system. more batteries and also a half decent inverter. With 12v you'd spend more on the fridge but less on batteries and panels and not need an inverter.
     
  11. armysnail

    armysnail Active Member Premium Member

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    It would be interesting to do a costing and see how long it would take to pay for the cost of the fridge. Beyond my knowledge level.
     
  12. stevo

    stevo Backyard Farmer Premium Member

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    early Christmas present for me, someone gave me two 6v 110ah batteries. Second hand, passed on from a business, they 're a few years old but are supposed to be ok.
    Because they're different batteries to what i have i shouldn't connect them to my current system, and I have a spare regulator and panel so i might set up a little stand alone system to run the pond pump.

    [​IMG]
     
  13. Mark

    Mark Founder Staff Member

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    Congrats Stevo and merry Xmas :) pretty soon you'll be running your whole place on diy solar... :cheers:
     
  14. stevo

    stevo Backyard Farmer Premium Member

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    cheers Mark,

    My main 240v fridge died yesterday which is ok as i have been using my 12v fridge but i wouldn't mind a bigger fridge, so i'll look around to see what's available.
     
  15. Steve

    Steve Valued Member Premium Member

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    Nice Stevo.

    What's the biggest size 12v fridge you can get? Are you talking about camping fridges or is there something else out there?
     
  16. stevo

    stevo Backyard Farmer Premium Member

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  17. stevo

    stevo Backyard Farmer Premium Member

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    just exploring options as part of the exercise...

    A 200L 12v upright fridge can be $2000
    A 70L 12v camping fridge is $1000

    Goodguys have a 200L 240v fridge $400. (i'm not sure how much power they need yet, 1.5 stars? amazingly bad efficiency?) http://www.thegoodguys.com.au/buyonline/LG_Electronics_205L_Top_Mount_Refrigerator_GN-205VW

    and then add a nice little 600watt inverter $340 http://www.batteriesdirect.com.au/shop/product/22654/ip600.html

    works out cheaper than a 12v fridge and gives me a nice inverter I can add to my system.
     
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  18. Mark

    Mark Founder Staff Member

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    That's a big savings but if the efficiency is really poor will it struggle to stay cold?
     
  19. Scott Mac

    Scott Mac Well-Known Member Premium Member

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    something that uses less than 300KW a year is a good place to start. If you trust the manufacturers specs. My Vest frost equipment is no where near as efficient as spec'd
    400L fridge and 250L chest freezer meant to use 1Kw/day, yet uses at best 2.5Kw/Day in reality and probably 3.6Kw/Day now we have kids. Too much for a 1.4Kw stand alone system.
     
  20. Mark

    Mark Founder Staff Member

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    Is Vestfrost meant to be a leader in energy efficient fridges?
    It's a shame you can't trust the specs because you'd think they would ensure the stated energy usage was spot on since it's so important when used with stand alone energy systems.
     
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