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UPS system

Discussion in 'General Electronics Discussion' started by screwball, Oct 11, 2012.

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  1. screwball


    Jan 9, 2012
    Hey, not a clue what section this would go in so mods please move if need-be,

    Basically Im looking at making my own uninteruptable power supply using deep cycle 12v batteries. Ive got an idea on the main of how the system would work however im abit unsure of calculations i may have if someone could confirm what im saying is true;

    I need to produce about 300w (230v AC so 1.3Amps?) to last 4hours
    so at 12v its (300w/12 = 25amps), so in total i would need 25amps x 4 hours = 100amp hours battery wise

    so to produce 1.3amps 230VAC i would need one 12v 100Ah battery?
    Obviously efficiency ratings haven't been calculated such as the inverter efficiency etc

    Any help would be great
  2. screwball


    Jan 9, 2012
    I plan to do this by:

    If there is supply, charge battery if not full
    if there is not supply, use relay to take the mains supply cables out of circuit of the batteries
    then that would turn the supply on for from batteries to inverter to the load
    when mains power is sensed that it is back, relay comes back into play, battery/inverter switches out of circuit and mains power charges battery again whilst powering load again

    Does this sound okay? its not for PC equipment or anything so it doesnt matter if it takes a short time (few seconds) for the UPS to come into place, unlike the proper UPS's take like 30milleseconds to switch over

    If im right it works out to be alot cheaper than buying a UPS,
    Biggest costs really are battery charger (£30) + battery (£120) + inverter (£30)
    A UPS providing these values are about £500 odd i think?
  3. donkey


    Feb 26, 2011
  4. donkey


    Feb 26, 2011
    thats about 250 au with postage higher. its new unopened direct from manufacturer but australian plug.
    my bad maths goes of a rough idea of exchange rates. your total price is roughly £180
    the uk gets 2 au for every pound i think so thats about right i think
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    OK, let's do some math.

    You're using the 12V supply to power an inverter that generates 230VAC. And your load is 300W. Let's assume that the efficiency is 80%, so you will be powering a 300/0.8 W load from your batteries (That's 375% -- but let's call it 400W).

    Also, depending on the nature of your load you may need an inverter rated for a much larger load -- fridges and freezers are particularly demanding like this.

    400W from 12V nominal power supply is 33.33A (let's call it 35A)

    So for 4 hours, you will draw 140AH from the batteries.

    Now, you NEVER want to discharge a lead acid battery completely. So let's go for a 50% discharge.

    Your battery needs to be around 280AH
  6. screwball


    Jan 9, 2012
    Thanks for all replies, its most appreciated i have decided to just invest in a proper one,

    If anyone has any ideas of something else i could make then that would be cool, im looking for something to do in spare time
  7. CocaCola


    Apr 7, 2012
    For a project using the same principles on a smaller scale aka learn those principle...

    Plan out small solar powered night light, that charges during the day and will illuminate all night... Work the math, build it and see if it works as expected, if not rework the math take measurements of the real circuit and see how to make it work...
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