Using a 12cm (12V, 6.8 watts) and 3 lithium ion cells from a used laptop batteries I made a powerful battery fan that lasts about 4 hours between charges.
Computer fan (12cm works best)
Batteries (used laptop cells, 3 cells for 12V)
2 fan covers
Step 1: Build the Handle
Step 2: Make the Battery
Wire the switch with the batteries.
Step 3: Finish and Charge
I use a 5V transformer to balance charge.
I also glued the switch between the handle sides so it is protected from accidental pushing. I positioned the switch so I could use my thumb to turn the fan on and off.