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A fan theory.

Toothless

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So my instructor at work is a pain and won't listen to me about me using my home-made USB desktop fans with the company computers, but said I can use a battery pack to power the fan.

By theory, using two 1.5v AA batteries to power a 12v DC, .25A, 3w fan, how long would the batteries last?

I know the fan won't be at full power (I need to find a way to link the battery packs together since it's two 1.5v AAs per pack, help would be nice) I'm just wanting air moved so my hands don't get icky sweaty while typing all day.
 
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Sorry cant answer your queation but wouldn't it be easier to buy a USB power bank that you use to charge phones/tablets.

The weight of the rechargeable battery should also keep it stable and then you can use your USB to recharge your battery.
 

Toothless

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I can get an 8-pack of AA's for $1 and I'll get a power bank at my next paycheck. I'm just putting in food for thought,
 
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Well, 1.5V AA batteries have capacity of around 2700-3000mAh (depending on grade). Usually those marked as "heavy duty" are best for short term, high drain devices (cameras, remote controlled cars etc). Avoid "longlife" types of batteries designed for long term, low drain devices that are suppose to operate for very long periods of time with tiny power draw (clocks, sensors, calculators etc for example)

Also, be aware that most computer fans have starting voltage of >4V, meaning 2x1.5V (3V) probably won't even start it up. You'll need at least 3x AA batteries totaling 4.5V. Sequential wiring.
 

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This fan spins up and moves enough air to my needs, the issue would be longevity.
 
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^
@Toothless ... A few of these together will give WAY more power then AA batteries.

These 18650 are 4.2 volts each and have a hell of a lot of power... These are the cells they put in lappys :rolleyes:

http://www.amazon.com/dp/B00NUI45AU/?tag=tec06d-20

You can also find the cell housings where ever online and @ 2500MAh per cell your golden my man :toast:


Don't do that you will forget about it and come back to a venting battery on your desk

Just get a cheap 12v dc powersupply, problem solved.
 
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Those universal ones with switchable voltage are the best. I have one to test fans outside the case. I can switch between 12V, 9V, 7.5V, 5V, 4.5V so I can test them for noise and functionality at specific spee without actually having them inside the case. They cost like 5 bags of peanuts...
 

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I'm gonna go ahead, break the norm, and actually answer the OP's question. Shocking, I know!

In order to figure out how much current the fan will draw, you need to realize that it can be viewed as a linear resistor, meaning Ohm's law is applicable.
Since we know from the fan's spec that it draws 250mA at 12V, we know it has a resistance of R=V/I = 12V/0.25A = 48 Ohm. It may be interesting to note that in this case it will draw P=V*I = 12V*0.25A = 3W.

Now, you want to "feed" the fan 3V. In this case, the current drawn will be I=V/R = 3V/48 Ohm = 0.0625A = 62.5 mA. It will consume around 0.1875W of power in this case.

The voltage delivered by the AA batteries isn't quite constant, it will drop with time and this may make the fan stop depending on the bearings and the minimum voltage it will operate on. The battery's capacity is directly dependent on the current drain placed on it, and from here you can conclude it will be roughly around 800 mAh. per battery. As the batteries are connected in series, each one "sees" the same current drain, and since you're using two, you can roughly double the capacity available. So at ~1600 mAh and ~62.5 mA drain, you can expect the batteries to last ~25.6 hours. I'd say 24 hours is more realistic, due to voltage drop near the end of battery life.
 
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A dc to dc step up converter circuit would allow the battery to keep the fan spinning for longer at a constant 12 volts at the expense of lower amperes of course.

You make me want to take out my electronics book again I rather out of touch for it. Like said above to calculate the total time the battery would last. You take the battery capacity in mAH by the mA the fan fan consume. Alternatively you calculate using watt hour. I use draw a diagram for an easier explanation.

Oh yes btw to use to 2 AA batteries at 1.5 volt to drive a 12 volt fan you need a dc to dc step up converter to convert the 3v of having 2 batteries in a series to 12v. If you want to avoid that you would need a battery pack that carry 8 aa batteries in series.
 
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Toothless

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I can't plug anything into the USB ports or wall outlets at work, just thought I'd toss that in.

A battery pack wouldn't be an issue since, 8 AA's for $1 would be fine, though I would think a 2200mha recharge pack would be easier.
 

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Sealed lead acid jelly battery would do the trick
Very Safe 6 or 12V ............capacity in Amp hours ( rechargable )


As found in Burgler Alarms fire Alarms Stair lifts for Elderly ect take one to work on monday fully charged take it home friday to recharge
 
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Something like this too simple? Spend a bit more for rechargeable AA batteries?
 

Toothless

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^ I was thinking something like that would work wonders since the wires are ready to tie and there won't be the chance to use any USBs.
 
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For a power bank I know what @fullinfusion means. That's how I built mine. I bought a cheap case from Ebay, I had a laying laptop battery around. Pried it open and took 6 x 18650 batteries. Used 4 in the bank. Works rock solid. Charges anything I plug into it and was wayyy cheaper than other options at the time. But your case is different. Here's some pics of mine:

IMG_0716_2.jpg IMG_0719_2.jpg IMG_0725._2.jpgIMG_0731_2.jpg
 
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^ I was thinking something like that would work wonders since the wires are ready to tie and there won't be the chance to use any USBs.

They are used a lot in the R/C hobby and are cheap as chips just about anywhere, and in any size/configuration you need.
 

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So are the laptop batteries like, able to fit in AA slots?
 
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Are these fans going inside of a chassis? If so, why can't you plug it into the PSU? If they're saying you can put it into the computer but not plug it into the PSU, then you must have a boss that is absolutely insane when it comes to micromanagement.

If you're talking about using them to cool the person sitting at the desk, I'm surprised that you're not putting your time to better use at work. Either way, I'm astonished.

Edit:
I should have read a little more.
I'm just wanting air moved so my hands don't get icky sweaty while typing all day.
Get a real fan that you can plug in. The company can't have you hurting yourself by using a computer fan incorrectly. I would also argue that it also means you're not stepping away from the computer often enough. Ergonomics is important.
 

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Are these fans going inside of a chassis? If so, why can't you plug it into the PSU? If they're saying you can put it into the computer but not plug it into the PSU, then you must have a boss that is absolutely insane when it comes to micromanagement.

If you're talking about using them to cool the person sitting at the desk, I'm surprised that you're not putting your time to better use at work. Either way, I'm astonished.

Edit:
I should have read a little more.

Get a real fan that you can plug in...

That was my idea but his dick head boss won't even let him plug a small fan in to the power strip.
 

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Get a real fan that you can plug in...

I can't plug anything into the USB ports or wall outlets at work, just thought I'd toss that in.
Asshole Boss
why dont you check to see if the supplied equipment is Certified by an electrician
Re Health and Safety should be Visably marked as such
 

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That was my idea but his dick head boss won't even let him plug a small fan in to the power strip.
So the boss won't let you plug anything in but will let you have exposed cables out attached to batteries and a unprotected fan whirling about? Sounds like an idiot (the boss) if you ask me. I couldn't tolerate that and would have lost my job already from yelling at the guy for nonsense.
 
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I would use a UPS battery ,standard ones are 12v7A/hour .

The Fan running at 12v0.35a should be able to run for a minimal 20Hours on one charge.

Battery 12v7ah
20 hour rate (0.35A, 10.5V)- 7Ah << Fan should fit into this Timeline
10 hour rate (0.68A, 10.5V)-6.8Ah
5 hour rate (1.13A, 10.5V)-5.65Ah
1 hour rate (4.56A, 9.6V)-4.56A

Price on a 12v7ah battery goes for +-R200 ($15)
Will need a charger .Takes 10 hours to charge battery via a standard ups

If could use wall outlet Then could of used a old cellphone charger and join to fan
 

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