battery bank question and how to improve it

Discussion in 'Off Grid Living' started by offgridbeginner, Feb 16, 2017.


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  1. ok so i thought of what i consider better than my last failed idea...

    doing a little more research and a few hours of lost sleep i think i have some what a solution

    i have a 2500w (5000 peak) inverter.
    Idea: hooking up my solar pannel to my 30A charge controller-- hooking battery up to that as well, and hooking up my inverter to battery... want to start running very limited things at first for example just my home office since it consumes most of the power... i have 2 desktop towers and 3 screens 45w 45w and 100w screen to be exact my pcu tower around 200 each...

    i do feel as though i am improving a bit on my rationality of what i want to power to be realistic.. but still remaining optimistic that i might find a better way in the future..

    question: how many 12v batteries would i need to keep it running for maybe 3 hours after sunlight and i get 4.5 hrs according to my zone.. im in florida.

    i will be charging my bank thru out the day with my 70W 18v solar panel for now but willing to see how i can also charge my battery bank after sunlight hours if that will be possible how so..

    AGAIN, Brainstorming for things i already have want to make it the most efficient dont wanna melt down anything here so thank you in advance again!
     
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  2. oil pan 4

    oil pan 4 Monkey+++

    Best case scenario a 70 watt panel will produce about 1/3 of a kilowatt hour, at the panel.
    Then the charge controller will be between 66% and 98% efficient.
    The cheap controllers are the least efficient.
    Then to get the battery to take a decent charge you need to charge it at between 5% and 13% of the 20 hour charge rate.

    A 70 watt panel on a cheap 66% efficient charge controler would be barely able to keep a 100 amp hour battery charged with hardly any use.

    For example to run just my refrigerator on solar it would take at least a $1,500 to $2,000 commitment. It would take about 700 watts worth of panels, two 12v 225 amp deep cycle lead acid batteries and a two 30 amp MPPT charge controllers and a 1000w pure sine inverter. Just to power a fridge that uses between 2 and 2.5kwh per day.
    Now scale that up to a house that uses between 10 and 30 kwh per day.

    Then you have power inverters.
    You really shouldn't be powering anything besides resistive loads with a cheap modified sine wave power inverter.
    Plus the cheap ones waste a lot of power while they are on with no load.
     
    Last edited: Feb 16, 2017
  3. i have 30A mppt charge controller 98% eff. .. im not getting at what you mean by the decent charging part...
    do you know how many i would need to keep my system from failing? i dont want to draw out more than im inputting (during sunlight hours) any solutions for night time?
     
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  4. TnAndy

    TnAndy Senior Member Founding Member

    The answers you get will only be as good as the information you provide.....nobody is going to sit and guess the amount of watt hours you need, and I'd say you don't know yourself. IF you're serious about doing something, get serious about data.

    Buy a "Kill-a-Watt" meter, plug your power strip you have all that stuff running off into the meter. Record your ACTUAL USE for a month (to get a good average), divide by 30 to get your usage in watt hours per day. NOW you have some real figures to work with.

    Bring that number back.
    Along with it, bring the brand/model of your inverter and charge controller.
     
  5. oil pan 4

    oil pan 4 Monkey+++

    Which MPPT charge controller do you have?
    There is a lot of garbage coming over from China that says MPPT just so you will buy it and it's not really MPPT.

    A decent charge is a full charge every single day the battery is used. That means a full round of bulk charging with out exceeding 110°F or the max recommended charging amps, a full absorbsion cycle per the battery manufacturer specs something like 14 to 14.4v for an hour or so and then float charge for some minimum amount of time, for example 14.2 to 14.5 volts for an hour.
    Some deep cycle manufacturers recommend up to a 12 hour of absorbsion and float charge, which is impossible on solar.
     
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  6. ghrit

    ghrit Bad company Administrator Founding Member

    Consider that your average use (from the Killawatt test) will not cover all your bases. That will describe very well the first step in sizing your battery bank. You will also need to know what it'll take to start the motor in (for example) your fridge. That may not be so easy to find, but there are some pretty good estimates that can be made.

    There's another thing you can do just because it'll be enlightening. Go around the house and read all the nameplates on the various appliances, add the wattage they call for to all the lighting you have whether on or off. (The parties need LIGHT you know --) Don't forget the well pump or macerator if you have one. Take all that and decide what you really can do without when the total is WAY off your expectations. Compare that total to the kwh shown on your electric bill divided by 30 (or whatever the interval might be.)
     
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  7. ok i have a rough number according to my FPL energy dashboard i am using 1kwh with ac- fridge- and a few things always running and that's no where near what i want to run... i only want to use it for my computer station at home.. (part of the 1kw/h )... i use 34kwh a day roughly. give or take 2 or 3 kwh depending on outside temperature because of my ac..

    again.. i only want this for my pc station it will save me a whole lot..
    the actual KWh for my station i dont have but im sure its less than 1 because that number is with fridge and a/c kicking in
    for my inverter and charge controller...

    inverter :

    Brand: COBRA
    Model: CPI 2575
    2500W (5000W peak)12V DC To 120V AC


    charge controller:

    CMTP02
    12v/24v
    30A
    ________
    Also: what will be best battery for this? 12v ?ah
     
    Last edited: Feb 16, 2017
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  8. ghrit

    ghrit Bad company Administrator Founding Member

    IF you are serious about powering ONLY your PC and peripherals, the job of sizing just got easier. Sum the wattages of all the loads you intend to power off the backup supply, then subtract the ones that will not be in simultaneous use. That is your starting point.
     
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  9. BTW I just ordered me a kill-a-watt meter to better explain my situations. i appreciate the help guys
     
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  10. UncleMorgan

    UncleMorgan I like peeling bananas and (occasionally) people.

    Something else to figure in when it comes time to calculate the size of your battery bank is the over-drain penalty.

    Evey kind of battery, even the "deep cycle" ones, will all die an ugly, early, death if you routinely charge them full and then run them flat.

    The smart thing to do is never drain your battery bank more than 50%. But even that hurts the tender little darlings.

    The wise thing to do is never run them below 75% charge.

    That makes your battery bank four times as large, and four times as expensive, but allows it to last about twenty times larger.

    A car battery can last several years because it's always being topped up by the alternator. So it seldom gets run down except when the engine won't start.

    The same battery, plugged into a kiddie scooter and repeatedly run flat, recharged, and run flat again, would probably be dead in about a week.

    Those electric trolling motors fishermen use are really quiet, but they purely eat batteries when used extensively.

    There is good news ahead though:
    Golden power: this super efficient battery is powered by a urine byproduct
     
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  11. i have
    -2 x 45W screens
    -1 32in. tv *100w energy guide tv
    -1 dell optiplex tower 760 *250W (rough) (it doesnt specify much)
    -1 dell small tower *200W
    -1 router *30w
    -1 modem *9w

    that is all...

    total watts =* 679W

    if i had 1000W flowing in here it will be great.. just in case i want to plug up anything small and simple i could.

    again my inverter is 2500W (5000W pk)
     

  12. my question exactly is.. how big does my batt bank have to be if i want 1000W ready to use every day.
     
  13. ghrit

    ghrit Bad company Administrator Founding Member

    That, even I (being mechanical rather than electrical) can answer. Taking 1000, multiply by 24 gets you to 24KWH. Now, to avoid running them flat, X4, gets you to 96KWH required in the cans on a dark and cold day. Oversized, probably, for your stated needs, but a bit of comfort and space to add stuff isn't wrong.
     
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  14. oil pan 4

    oil pan 4 Monkey+++

    So you need roughly a 3,000lb forklift battery. Just to get 24kwh, but only 12kwh would be use able. Because you don't want to run them down more than half their capacity.

    If I was going to go off grid I would get 2 of the biggest 24v forklift batteries I could find, connect them in series and run a 48v system.
    48v @ 1,000ah would be a good start.
     
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  15. techsar

    techsar Monkey+++

    Sloth is spot on...no way is that $15-20 controller going to do any favors for your batteries, even though it is marketed as MPPT.
    But you already have it on hand...use it and inexpensive batteries as a learning experiment. Make sure you use a power strip so you can shut your computer system down ALL of the way. Ghost loads will sap a lot of power if you don't.

    BTW, Welcome to the forum! Lots of knowledgeable folks, trying to guide you from making some common mistakes :)
     
    Last edited: Feb 16, 2017
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  16. oil pan 4

    oil pan 4 Monkey+++

    I highly doubt that a $20 charge controller is MPPT.
    The cheapest one of know of that is a real MPPT charger is the genasun 5 amp for LiFePO4 batterys, it would also work well with gel cell lead acid. It's about $90.
    I have the larger 10 amp genasun for LiFePO4. Mine is almost 99% efficient with 22ocv mono panels.
    The smallest one I would recommend for lead acid is the Morningstar 15amp, it's about $225. I measure it at around 98% efficient with 21ocv mono panels.

    I did not know forklift batts like to charge at 30% of C. Where I work the fork lift charger peaks at about 15% of the forklift battery's C rate and doesn't stay at that power level real long. (An 800ah pack charging at 125 amps)

    I gave away and sold all my cheap modified sine wave inverters years ago.
    On account of them being no good for most electronics.
     
    Last edited: Feb 16, 2017
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  17. THANKS i do appreciate it!
     
  18. Tevin

    Tevin Monkey+++

    1000 watts at 12.65 (nominal) volts is 80 amps (rounded).

    So assuming you are working with a 12 volt system, you need enough battery to push 80 amps for as long as you want to draw 1000 watts.

    Even for short term needs, that's a hell of a lot of battery.
     
  19. thats my issue so far.. i dont know which typ eof battery would be better what ah rating should i get and how many 12v batteries
     
  20. oil pan 4

    oil pan 4 Monkey+++

    From now on ask questions before you buy anything.
     
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