How best to run a mini fan for at least 4 hours on battery?

I'm doing a project where I'd like to run a small 25mm fan for at least about 4 hours (the max length of my requirement) before having to change the battery. The fans I have found which would suit my purpose are usually 5v or 12v. Please tell me which voltage you would recommend and how best to power the fan with the smallest types of common batteries if possible (space is limited). Thank you very much! (I'm smart at some stuff but totally ignorant about this stuff, so don't be afraid to break it down for me.)

5v fans are available that draw 120mA. A 9v battery is rated at 500mAh, so with a simple dropping resistor, you can have the fan running for four hours.

if you add a step-down dc-dc converter, you can get 6 to 7-hours out of the battery.

Thank you Most Helpful Member for your quick answer . Could you please explain how to add a dropping resistor.

If you are using a 5v fan that needs 120mA and powering with a 9v battery, you need a resistor that drops the voltage from 9v to 5. That means, you need to drop 4 volts.
Using ohms law, resistance = voltage / current = 4v / 0.120 amps = 33 ohms. Luckily, 33 ohms is a common resistor size. You'll need a big size (physically big) to dissipate the heat. 0.120 amps x 4v is 0.5 watts. You should get a 1 to 3 watt size resistor. The most common is by far is 1/4 watt so the one you need will look much bigger than you're used to seeing.

A better option is to use a 5v linear regulator (LM7805). This is a common device made by many companies. Connect battery to input (+) and ground (-). Connect the fan to the output pin (5v) and ground (-).

even better option, (but more expensive) is a dc-dc converter. This one, lets you use a 5v usb phone charger to charge a lithium battery (3.2 to 4.2v depending on level of charge). Then the same board will give you 5v output no matter what level the lithium battery is at. It is a buck/boost converter.

Buy this board, a battery from the same company so you get the right connector connected the eight way and you can recharge as often as you like. I just stumbled upon these boards myself and it works great. You will need to know how to solder though (with all of these options).

adafruit dc dc converter

33ohm resistor

7805 regulator

I suggest that you run a 5 V fan from a powerbank. Powerbanks contain 4.2 V lithium batteries and the step-up converters to give you 5 V.

Diver300 said:

I suggest that you run a 5 V fan from a powerbank. Powerbanks contain 4.2 V lithium batteries and the step-up converters to give you 5 V.

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Also a good idea! A 8000mAh could run the 120mA fan for several days nonstop for $15 to $25 range.

So if I use a powerbank, the only ones I've seen use a USB cable. Would I cut the USB cable and connect the cut and stripped USB wires to the 2 fan leads? Which wires in the cable would I connect? Aren't there several? Thank you all again.

There are 4 wires in a USB cable. Often the power ones are red and black for + and -, and they are often fatter wire than the other ones.

The + and - ones will be the only ones that have 5 V between them.

If you connect a simple fan to a power bank and get the connections wrong, it just won't work. You are very unlikely to damage anything.

I agree with the idea of a power bank, and they are available ludicrously cheaply.

My only 'issue' with them is that they shutdown (turn OFF) if the load is too small (so most PIC circuits won't work), but for a 5V fan they should be absolutely great.

Nigel Goodwin said:

My only 'issue' with them is that they shutdown (turn OFF) if the load is too small (so most PIC circuits won't work), but for a 5V fan they should be absolutely great.

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Sorry Nigel, your blanket statement of "fact" is incorrect. Fancy power banks, usually with multiple cells, do often have this problem.

On the other hand, many single-cell non-fancy power banks do not. If there is not a push button power switch, this type always provides 5v to the USB connector (as long as the battery isn't flat). I have used this feature for a customer who wanted an instrument for training without the hassle of finding an outlet.

billionj said:

So if I use a powerbank, the only ones I've seen use a USB cable. Would I cut the USB cable and connect the cut and stripped USB wires to the 2 fan leads? Which wires in the cable would I connect? Aren't there several? Thank you all again.

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It typically looks like this. There may be two black wires. The two blacks may be connected to each other but if one doesn't work, just try the other. Like a previous poster said, you ain't going to break anything so go ahead an experiment

you can also order the wire already stripped for a few bucks but, must people have an extra laying around.

The color code in a random USB cable may be meaningless or even wrong. So if (what you think is) positive to red and (what you think is) negative to black doesn't work, try swapping them around.

Visitor said:

Sorry Nigel, your blanket statement of "fact" is incorrect. Fancy power banks, usually with multiple cells, do often have this problem.

On the other hand, many single-cell non-fancy power banks do not. If there is not a push button power switch, this type always provides 5v to the USB connector (as long as the battery isn't flat). I have used this feature for a customer who wanted an instrument for training without the hassle of finding an outlet.

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I've got a fairly large collection of power banks, including many single cell ones - ALL disable the 5V converter under insufficient loads. It's a standard feature, as it would soon run flat if the converter was running at all times. I've never seen one which doesn't do this, although I'll happily admit there could be some poorly designed old ones which perhaps don't?, or poorly designed homebuilt ones?.

I'd imagine pretty well all use the same Chinese IC's?, so have the same functionality.

Number of cells doesn't really make any difference, as they simply parallel the cells anyway, so the more cells the longer they last, and the longer they take to charge.

Once again, you deny that the experience of anyone else is valid if it doesn't match your own. And once again, you are wrong.

It's a pity that you don't seem interested in helping people here?, your only reason to be here seems to be attacking people who are trying to help. You claim my experience is 'invalid' yet seem to believe that yours is?.

You also don't give any suggestion of the power consumption that you 'claim' your single example was using? - so it's totally meaningless. Quit epossibly it was high enough to prevent auto-off?.

I'll repeat again, in my experience across a reasonable number of different power banks, they all auto-switch off after a few minutes if you try and use them to power low current circuits (such as most PIC circuits) - as presumably they all use the same IC. This applies to all the single and multiple cell designs, and to ones with and without buttons, that I have tested.

I appreciate that there 'may' be some which don't?, but I've yet to see one, and they wouldn't last very long if they were permanently running the converter - so would tend to be rare.

Personally I'll go with the vast majority, and not the odd one that 'might' possibly work at low powers.

Nigel Goodwin said:

You claim my experience is 'invalid' yet seem to believe that yours is?.

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visitor claimed that both options are possible depending on the specific brick, why you Nigel Goodwin, keep harping on only your own "I-must-be-correct-experience-because-I-have-a-one-data-point-trend" to prove it. Reading your comments is like watching a YouTube compilation video of skateboard accidents.