I have a computer fan that I am using for a school competition. The rules state that voltage used can be no higher than 9 volts but most fans are 12 volts. The fan that I have is has a recommended 12 volt 4.5 amps. So if I used a 9 volt battery or 7.2 volt how many amps should that battery be? How do volts effect amps in motors? I am new to all this and so a simple answer would be appreciated. Also I need it only to run for about 10-15 minutes at most so if you have any battery recommendations that would also be helpful. The rules for batteries are no lithium or lead allowed. The rules are very prohibitive.

Newly Added Information

I am building a small scale hovercraft that has to be as light as possible. However I am awarded points for amount of weight it can carry. I am using this fan to fill the hovercrafts skirt. I have to use a computer fan because the rules state I can only use brushless motors if it is in a computer fan otherwise motors have to be brushed which I have had issues with. I found this fan in my house and found it very powerful. Here is a link https://www.digikey.com/product-detail/en/delta-electronics/PFR0912XHE-SP00/603-1707-ND/3078695 Here is a full copy of the rules its under hovercraft http://api-static.ctlglobalsolutions.com/science/SO_C_2018FINAL.pdf

  • \$\begingroup\$ the amps will probably decrease along with the fan speed ... but why are you even asking? you are the one who has the fan. just test it out. ... that is part of the development process in competitions. \$\endgroup\$
    – jsotola
    Dec 30, 2017 at 22:09
  • 5
    \$\begingroup\$ 12V 4.5A, that is 54W. For a computer fan? Are you cooling the ENIAC? \$\endgroup\$
    – dim
    Dec 30, 2017 at 22:14
  • 4
    \$\begingroup\$ So if I used a 9 volt battery Forget about a 9V battery and your 54 W fan. You'd need about 100 standard 9 V batteries to get that fan going. A 54 W fan isn't something you should be experimenting with. Get a small 12 V 0.1A fan and use that instead. \$\endgroup\$ Dec 30, 2017 at 22:24
  • 1
    \$\begingroup\$ what is the project that you are building? \$\endgroup\$
    – jsotola
    Dec 30, 2017 at 23:24
  • \$\begingroup\$ the only way to push in more amps on a fan is to raise the voltage, which you can't do. PC fans can run on as little as 5v; a common silent pc mod is to run it off 7v (+12v to +5v)... \$\endgroup\$
    – dandavis
    Dec 31, 2017 at 16:15

3 Answers 3


a simple answer would be appreciated


if I used a 9 volt battery or 7.2 volt how many amps should that battery be?

You clearly mean ampere hours.

If it uses 4.5 A when being fed 12 V, then you can assume that with 9 V battery that it will drain roughly \$4.5\text{ A}×\frac{9\text{ V}}{12\text{ V}}=3.375\text{A} = 3375\text{ mA}\$.

Let's assume you will run it for 15 minutes, then you will need a 9 V battery with at least \$3375\text{ mA}×\frac{15\text{ min}}{60\text{ min}}=843.75\text{ mAh}\$.

Considering that you will be discharging it at 4 times the capacity, then I'd aim for a little bit higher capacity, say 2000 mAh or even higher, or put several 9V batteries in parallel. Because high discharge (several amperes) will lower the capacity.

How do volts effect amps in motors?

If you increase the voltage, then the ampere will also increase.


Computer fans increase load current with voltage above the start threshold.

Most PC fans are <= 5W so I suspect you have an error in the decimal place.

enter image description here

from the red graph line I expect your case fan is identical to this with 450mA at 12V and 300mA at 9V


The amps will for an ordinary fan be proportional to the voltage ie. it will decrease with decreasing voltage. not all fans can run at lower voltages than those specified but you should consolidate the datasheet. any way you should expect the fan to run at a lower rpm.


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