I've done a few experiments, and as you'd expect the specs aren't 100% precise, you need to input a bit more than 3V to get it to work. But, with 4 NiMH batteries (5.2V total) it works quite well.
One of my tests was to drive one of those 12V computer fans off 4 C batteries. Worked just fine. The draw on the batteries once it had spun up was about 400 mA, the fan sticker says 190 mA consumption but I assume that you can use lower input voltages but of course you'll get more current draw.
When I plugged in two fans, they spun, and the converter started making this high pitched squeaking noise. At first loud, few seconds after spinup it got quieter, but it was still there. Checking the draw on the batteries, it started out at about 1400 mA, then went down to about 1100 mA. I switched to one of those square 9V batteries, the squeaking went away.
The converter is rated for 2A output, so it should have no problems running those two fans. But, why the squeaking noise? Is it normal on those circuits, or is it a sign that I'm pushing it past its limits?
The ebay ad gives no rating for the INPUT current, and that is likely to be limiting when you are stepping voltage up. I would guess the input current, like the output current, cannot go much above 2 A.
Keep in mind that the efficiency will go down substantially when there is a large difference between the input and the output voltages (e.g. to 50% or so) and lowered efficiency increases the input current, for a given output power.
mmelendeze:
The converter is rated for 2A output, so it should have no problems running those two fans. But, why the squeaking noise? Is it normal on those circuits, or is it a sign that I'm pushing it past its limits?
Is it getting hot?
The next step is to look out power (volts x amps) on input and output. See how much efficiency you're getting.
The current is fundamentally limited by the inductor's saturation, which in
this circuit will mean the maximum current is limited. The heat generated may
be excessive if you go close to the red-line, and a small cooling fan could be
needed.
Yes ferrite's squeal, unless operated in the ultrasonic range (when they annoy pets),
but the sound volume will increase a lot as the upper limit is reached I suspect. Two
effects lead to vibration in the inductors, magnetostriction (the ferrite changes
shape slightly on magnetic reversal), and winding vibration (think of the windings
as like a voice coil in a loudspeaker).