Need help solving battery mAh question..

[imac567]

I'm a beginner and need some help, here's the question, if I connect a 9v energizer alkaline battery to a 12v micro fan rated at .25 Amps, how long (in theory) will the fan run? I believe the battery is rated at app. 595 mAh. If you can solve this please tell me how you did it. Thank you.

 

[(*steve*)]

This sounds like homework. Is it?

first, you could assume the motor has a simple resistance.

Then you could determine that resistance using the current at 12V.

Then you could determine the likely current at 9V

Then you could calculate how long the battery would last.

Your answer might be quite optimistic because the small battery will not provide that capacity at a high current. The datasheets for the battery may give you better data for higher currents. In addition the voltage will fall over time and the fan is almost certainly NOT a simple resistive load. All of these things will conspire against you.

 

[imac567]

Thank you for the help.
No, it's not homework (I'm too old for that) lol. I'm making an air purifier that uses a micro fan to pull the air in a closet through a mesh with activated carbon on top of it. Since there isn't any power in the closet, I decided to use batteries instead.
Concerning the micro fan, can you check the resistance across the leads or is it the windings that are making the resistance vary? I thought I could calculate the amps but I'm getting .640mΩ which makes the amps 14, that can't be right.

 

[duke37]

It can be right.

The motor generates a back EMF when it is turning which is proportional to speed. Thus a motor running on 12V may be producing 11.9V back EMF. The current taken will be 0.1/R. The slower the motor runs, the less the back EMF and the more current.