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Sizing battery for solar system

Discussion in 'Renewable Energy' started by vinke, Feb 6, 2008.

  1. vinke

    vinke New Member

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    Hi everybody,
    Im having some troubles with the calculation of the battery size for my solar system. I have two small dc water pumps, 30W each.(12V,2.5A).
    The two pumps work independent on each other and can be activated randomly. That, both may run at the same time. That would be the worst case situation and they will draw 5amps from my battery.assuming that any of the two pumps would run more than 30 minutes,how do i calculate the minimum battery Ah required, so that the load can be supplied when there is no sunlight without the battery gets drained too much.

    Also I am incorporating a low voltage disconnect circuit in my system,to cater for the eventuality that my pumps runs longer and drains my battery. I would like some advice on how to calculate the low voltage disconnect point.I am perplexed because while on load the battery voltage may drop much below its actual voltage.
    Some ideas and advice would be most welcomed.
    Thanks
     
  2. Hero999

    Hero999 Banned

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    What sort of batteries are they?

    Lead acids normally shouldn't be discharged below 10V.

    You've not said the maximum amount of time the motors will run for or how much power the solar cells under normal weather conditions in your area.
     
  3. HarveyH42

    HarveyH42 Banned

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    Just guessing from what you've posted, you'll need a good size battery. Thinking a deep-cycle marine type battery, and a pretty good size solar panel to go with it. The pumps will be running day and night, correct? But you may not get much charging done during the day, also the panel will need to provide more then what the pumps consume in a 24 hour period, just to keep the battery charged.

    Depends a little on where you live, and how many hours per day you can expect the panels to be exposed to full sunlight per day. I figure on about 6 hours here in Florida. Never said what the pumps where used for, if it's very important they keep running, need to make sure the battery(s) can last a day or two without getting much of a charge, suppliment from the outlet.
     
  4. dave

    Dave New Member

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  5. rjvh

    rjvh New Member

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    your batterys are to small in capacity or they are old and almost gone

    the baterys you need is deep cycle ones
    as sugested earlyer marine batterys will do electrical wheel chair batterys are also good

    the size you should have is depending what you do

    you mentiont 5 amps for 1/2 hour maximum on a day

    take that tripple (safety factor of 3) = 7.5A/h
    Cable losses,
    Tempature differances from your battery will effect greatly the capacity (best tempature is about 20 to 21 Degree Celcius)

    assuming that you have at least sunlight for 6 hours
    and on a cloudy day still be able to get 40% of the capacity from your pannels

    7.5A * 12V = 90

    90W : 6hours = 15 W/h

    15W * (100 : 40) = 37.5W that should be your capacity of your panel

    this only a guid line and you will be able to tweaq this system furter

    there are many factors still not compute in this but this gives you at least a workable set up

    solar panels tend to be more efficient in colder climates
    tempature of the battery is a big factor

    i also atache a schematic of a solar (dis)charge controler

    i did build this one my self and it works fine
    there is a battery low voltage protection in it a short circuit protection and a charge controler incorparated in this circuit

    Sucsess
    Robert-Jan
     

    Attached Files:

  6. vinke

    vinke New Member

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    Lead -acid type. I did said that the motors should not normally run more than 30minutes per day. They run independent from each other. That is one can be already be running then the other one start running.The worst case situation is if both start running at the same time for 30 mins .they will drain 5amps from the battery during 30mins.
    I live on a tropical island, with average 10hours of sunlight per day in summer and 8hours in winter.
     
  7. vinke

    vinke New Member

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    Thanks rjvh. it reli helped a lot. Thanks for the schematic,though its in german( i think its german.lol), I understood the principle.
    Is the method u described above for sizing battery and panel a certified one, I mean taken from a book or from research?
     
  8. rjvh

    rjvh New Member

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    the shematic discription is not german but dutch

    if you living on a tropical island than keep in mind that your panels are less efficient deu to the heat but NR of sun hours does compesate that

    your batterys will be warmer than 20 degree Celcius so your charge voltage for the batterys have to be a little lower than normaly is the case

    about the methode it's a practical one which give you a start set up

    this methode is a rule of thump and i did find it out by trail and error with the basics of from data from battery suplyers

    I calculate also back up times for for switch board control panel and alarm systems like this and it leaves me alsways with enough safety marge

    we still have a lot of power cuts here

    by the way if you have that amound of sun hours on your island than it's about the same distance from the equader as cambodia so i assume that the conditions are the same as here

    you will get a way with an even smaller solar panel

    Robert-Jan
     
  9. Hero999

    Hero999 Banned

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    That's 2.5Ah per day.

    Lead acids require something like 25% more current put in than you get back so that's 3.125Ah.

    At worst case you get 8 hours of daylight a day but solar panels are specified for strong sunlight so in the winter you probably only get a couple of hours at full power output so you better divide that by two giving 4 hours.

    3.125/4 = 781.25mA

    The nearest size solar cell you can get is probably 800mA or 1A.

    EDIT:
    I've just realised you want to know the size of the battery. A 1h would do but I don't think you can get lead acids that small and if you can they're not cheap. Go for a 4.5Ah battery or just the cheapest lead acid battery you can get hold off.
     
    Last edited: Feb 8, 2008
  10. mneary

    mneary New Member

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    Your battery should be sized for the longest period you want it to run without sunlight. If you're using 2.5AH per day, and if the sunlight might disappear for 4 days, you need a total storage of 10AH. (Hero may have intended to type 10AH).

    Similarly, your solar cells need to be sized according to how long you want to allow for restoring the deficit from dark day(s). If you want to restore it in two sunny days for each dark day*, then Hero's suggestion of 1A will do it.

    *after 2 dark days, the battery is fully recharged after 4 more sunny days, etc.
     
  11. vinke

    vinke New Member

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    I have a 7.2Ah lead acid battery already available, thats good since the minimum battery size is about 3Ah.
    So the minimum solar panel would be one giving out 1.8A if i assume 4hours of minimum peak sunlight.That is a 30W(17*1.8) panel would do. Thats great coz i already have a 75W panel. The system will be oversized but that doesnt matter because its not for long term use, just for demonstration.
     
  12. Oznog

    Oznog Active Member

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    Actually you need a charge controller for sure. Otherwise the panel will continue to deliver current into a full battery, although at a lesser rate. This overcharge will dramatically reduce the battery's life.

    Plenty of charge controllers out there for large and small systems. You need to size it according to panel size, not battery size.
     
  13. Hero999

    Hero999 Banned

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    Just limit the battery's maximum voltage to 14V.

    An easy way to do this would be to use a zener diode. A better way of doing this would be to use a comparator and reference connected to a MOSFET to disconnect the solar pannel when the voltage exceeds 13.8V and reconnect it when it falls below 12V.
     
  14. vinke

    vinke New Member

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    I am regulating the panel output voltage to about 14V by making use of an adjustable voltage regulator, the LM338. It has guaranteed 5A output current. The drop out voltage across the regulator is about 2V plus the voltage drop across a protective diode( preventing current from flowing back into the panel) this makes roughly 3V voltage drop in the worst case. Which means that my panel needs to be in peak sunlight for its output voltage(17V max) become sufficiently large to overcome these drops and provide the necessary charging voltage.
    I am using current sensing to monitor the battery state of charge. When the charging current drops below 0.2A, this will cause the panel to be disconnected from the battery.
    To reconnect the panel, the battery voltage is monitored.When the latter drops below about 12.4V the panel is reconnected back to the battery.
     
  15. Hero999

    Hero999 Banned

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    You could use a low droput regulator to make more use of the solar pannel.

    Don't worry about the battery overcharging, the circuit you're using will already prevent that.
     
  16. retro

    retro New Member

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    I shelled out $250 for a 40 AMP, MPPT Charge Controller. It's very easy to overcharge your batteries. Decent Charge Controllers aren't cheap, but then neither are deep cycle batteries.
     
  17. Krumlink

    Krumlink New Member

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    Easiest ting I can imagine is using a car battery, they are rated 12V 100+aH right?
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    But they aren't rated for deep cycle usage, so will tend to have a fairly short life span.
     

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