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How to Build a Battery Back up for the Blower?

Post in 'DIY and General non-hearth advice' started by Srbenda, Sep 14, 2010.

  1. Srbenda

    Srbenda Burning Hunk

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    My insert has a 8amp, 115v, 60hz blower motor, and I want to build a relatively small backup power supply for this motor.

    I have zero electrical engineering knowledge, so how do I calculate how much battery power I need to run the motor for approximately 8 hours?

    I would like to have a setup that would keep the battery on a charger, and during a power outage would be moved to the blower motor with an inverter.

    Any help?

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  2. 73blazer

    73blazer Member

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    8amps*110v=880watts.
    You'll need an 880watt inverter, probably more like 1000 because it's a motor and takes a startup surge load.
    Then you just need to pick enough batteries to to run 880watts for your specified period of time.
    Is that what the motor is rated at or have you ammetered it out and find it's running load? Running load is probably a bit less than rated. But you never know.

    If your going to keep the batteries in the house, you'd need nice sealed batteries, like Optima's. The biggest deep cycle (blue top) optima they make is 75 amp-hours (D31).
    Meaning, it'll run 75amp load for 1 hour at 12v.That's 900 watts for 1 hour. FOr your situation, inverters are usually only 80% efficient at converting DC to AC power. So your actually wattage for an hour is 720. So you'd probably need 2 Optima D31's to really run your motor for an hour or slightly more.
    Then your going to have to charge and keep charged those batteries, so you can either get an inverter with a built in charger. Or have a separate battery charger and keep them topped off at least once a month.

    In all your costs involved would be something like:
    2 Optima D31's (approx $200/each): $400
    1000watt inverter : $120
    or 1000watter inverter/10amp charger: $200
    misc 8 gauge wiring : $20

    So your in for $520 at a minimum and possibly more if you don't already have some other means to charge batteries and alot more if you want to run it for more than an hour (you'd need 2 batt's for every hour of operation, roughly).

    I supposed you could disassemble the fan motor unit, because it'll have a giant AC to DC converter built into it (most all small motors run on DC anyway), if you wired the motor direct to the battery, you could not lose the 20% both ways and lose the cost of the inverter, so effectively a 40% gain on the system. But then you'd have to wire in a switch to be able to run it on normal AC.

    My advice, buy a small gas Generator, far more useful for other things and less maintenance.
  3. ANeat

    ANeat New Member

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    Thats a lot of power to ask from a battery or several batteries.

    8amp @ 115volts is 920 watts, now thw actual draw may be less than that.

    To get 920 watts out of a 12 volt battery is 76 amps. Thats a lot of current for 8 hours.


    If you insist on battery backup higher voltage will reduce the current load. 2 12 volt batteries in series (24 volt) would cut the current in half, 3 (36 volt) would only need 1/3 the current (25 amps) but the appropriate inverter would be required


    A small generator might be a better solution. 1kw or 1.5kw.... Or just be happy with the radiated heat and live without the blower if the electric is out
  4. ANeat

    ANeat New Member

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    +1 to what 73blazer said, he typed it faster than me :lol:
  5. 73blazer

    73blazer Member

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    Even better! :)
  6. Srbenda

    Srbenda Burning Hunk

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    Wow.

    I thought a couple of batteries, and an inverter would do it.
    I have a generator, I guess I'll just use that.
  7. 73blazer

    73blazer Member

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    I think the real issue is that 8amps. That seems really high for an insert blower. I bet if you amped it out it's really only pulling 3 or 4. But, you just don't know unless you put a meter on it.
    STill, if it was 4 amps, those 2 giant optima's would only run it for 2 hours or so. Still $520 for 2 hours runtime, plus charging (inverters w/built in chargers cost alot more).
    I think Generator is best route!
  8. btuser

    btuser Minister of Fire

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    8 amps is a hell of a draw from a blower. Are you sure that's not .8 Amps? With that kind of fan you'd have to tie down the furniture.
  9. Srbenda

    Srbenda Burning Hunk

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    Here's what I am basing it on.
    Do you see 8A?

    [​IMG]
  10. ANeat

    ANeat New Member

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    looks like .8 to me
  11. Srbenda

    Srbenda Burning Hunk

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    Yep, I just looked at the original photo, and it's .8A

    This should make the calculations different!
  12. ANeat

    ANeat New Member

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    Yea instead of trying for 1000 watts you only need to worry about 100 watts
  13. Treacherous

    Treacherous Minister of Fire

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  14. 73blazer

    73blazer Member

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    So, one Optima D31 would run it for approx 7-8hrs.
    Still need the inverter/charger. But, far more feasible@0.8amps!
  15. KB007

    KB007 Feeling the Heat

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    You could prolly buy a few hundred or 1KW UPS for a couple of hundred bucks
  16. billb3

    billb3 Minister of Fire

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    even with a generator, you might want to protect the stoves circuit board from voltage and hz variants from the generator with a UPS.
  17. Slow1

    Slow1 Minister of Fire

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    That's the route I'd suggest - watch the sale fliers or the net. They come up periodically from office supply stores etc. Off brand UPS systems etc sometimes for amazingly low prices. With the UPS you will get the automatic failover as well (a bit harder to wire in your home made solution). Now the question is how long do you want to be able to run it for? I would think that if you get 30-60 minutes out of it you are fine, any longer than that go hook up the generator that I think you said you already have, right?
  18. oldspark

    oldspark Guest

    If you go with a UPS make sure you get the right one as some of those do not like to run motors.
  19. Treacherous

    Treacherous Minister of Fire

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    Agreed. You would definitely want to utilize one of the inverter generators from Honda, Yamaha, Kipor, etc.

    I would never use a standard square wave generator with electronics.
  20. oldspark

    oldspark Guest

    Not sure I understand what you are saying , you are not going to have a square wave with a motor driven gen.
  21. Treacherous

    Treacherous Minister of Fire

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    I should rephrase... it is actually a squarish wave in the cheaper non-inverter based generators versus a pure sine wave in an inverter based generator.


    I don't claim to be an expert but that is what I am told.

    The Hz difference alone will make a PC clock skew fairly quickly.


  22. oldspark

    oldspark Guest

    A motor driven gen. puts out a very good sine wave not squarish at all so you may be mixing things up a little.
  23. Treacherous

    Treacherous Minister of Fire

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    I may have been thinking of an old 12V inverter that did a square wave but isn't the sine wave in a non-invertor based generator called a "modified sine wave"?

    I find that I am able to power up full size refrigerators and freezers with my EU2000i that choked my old 1850 Coleman Powermate.

    [I apologize for taking this somewhat off topic]
  24. oldspark

    oldspark Guest

  25. cycloxer

    cycloxer New Member

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    That's only .8 amps. A small computer UPS is the easiest method. Of course, this all depends on how long of a backup you need.

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