I am pretty handy but electronics are not my thing.

I have one man tent I use for backpacking but since its small, the condensation builds up on the inside badly. I want to build a small fan to help with air circulation.

I have a computer fan rated at 12v .24A. I also have a portable phone charger rated at 5.5v 800mA. with a 2600 mAh battery in it. It runs nice and silent on this lower voltage. First test was about 4.5 hours run time. How do I calculate power consumption of the fan at a lower voltage? OR how big of a portable charger do I need for 8-10 hours of runtime plus a little extra for a LED light for a while?

 

You would need to measure the current the fan draws at the lower voltage.
Or charge the existing battery fully and see how long it will run the fan for.

 

You would need to measure the current the fan draws at the lower voltage.
Or charge the existing battery fully and see how long it will run the fan for.

I was hoping there was a way to calculate the current draw based on the lower voltage vs. the specs of the fan.I could wait for it to run out but I have to actually be there when it does...hours later.

 

It all depends on the characteristics of the motor. Even a datasheet for the motor is unlikely to show performance for much lower than rated voltage.

 

It all depends on the characteristics of the motor. Even a datasheet for the motor is unlikely to show performance for much lower than rated voltage.

That makes sense. However, as a rough guideline, if I take the 12v .24A motor and run it on half the voltage, may I expect more or less half the power consumption?

Also, it is a 6 blade fan. May I presume that if I cut off half the blades and reduce the load by half, it will reduce the power consumption comparatively?

Just kinda thinking out loud on how to make my existing parts run twice as long...

 

Buy a different tent with better ventilation and/or setup your tent properly.

https://sectionhiker.com/how-to-prevent-tent-condensation/
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Hello there,

A rough guide with no data and no measurement would be half the current at half the voltage. That's if the fan will start at that voltage too.

So if 12v and 240ma, then say 6v at 120ma.

The real trick is trying to compute the run time with a given battery without having the spec's of the battery. The ampere hour rating is usually based on the C/20 rating which means that is the measured capacity if you discharge the battery over a 20 hour period. So for a 800maHr battery 20 hours would be 40ma, and since you will be drawing around 120ma, that is 3 times as high. IF there was no change here we would get 20/3 hours run time, but there will be a change due to the higher current so it will be less than 20/3. Perhaps 20/4 which is 5 hours, or 20/5 which is 4 hours. This must be tested because there are too many unknown variables though.
Also, as the battery ages the run time will become less and less until it runs down much faster.

I run a high power 12v fan now and then at less voltage. It even runs at 5v but the current is less than computed above.

 

Thank you, This pretty much answers my questions