Review on Stay Cool Anywhere: O2 Cool 5 Inch Battery Operated Portable Clip Fan (2 pack) - Grey by Dawn Valdez

Two D-cells create a gentle breeze off a treadmill or elliptical rack, but I wanted more. I bought two Trustfire 26650 3.7 volt 5000mah batteries. Of course, this required a charger, and the Nitecore Intellicharger i4 is one of many that work. You should be careful to get Trustfire lithium cells with a built-in protection circuit that shuts off the current if the voltage falls below 2.5 volts. This prevents damage to the lithium cell, which cannot tolerate deep discharge. The 26650 cell is approximately two inches shorter than the two D-cell stack. Connect the gap with a 1/2-inch #1 15/16 bolt and nut. Wrap it with tape so that it stays in the center of the battery compartment. Make sure the screw extends two threads past the nut to make contact with the positive fan connector. In the picture it should be clear. This does not require changing the fan, and the resulting breeze is now quite strong. Several pieces of double-sided adhesive foam on the bottom form a groove that secures the fan to the edge of the treadmill shelf. Or stick on some sticky soft rubber pads to keep your phone from sliding on your car's dashboard. I've run this system for many hours and the fan handles the extra voltage well. A fully charged battery should last 5 to 8 hours.