Once you have a satisfactory generator setup, then you want to connect the generator to the charge circuit, insert the battery, and use your multimeter to test the output voltage at the USB port. If you don't see 5V then there is a problem. Fix it before plugging your pricey gadget into the USB port.



Below you can see my assembled breath powered USB generator in all its glory, top and bottom. You can see the rubber band used for return, along with the linear gear carriage, the strap and the paper clip I used to connect the linear gear to the strap. The key here is to have all motion transferred to the linear gear so you want the strap and connection method to be stiff with no give. The strength of the rubber band or spring return is up to you. My half-assed experiments indicate that you can pretty much handle a 1N force without feeling too labored in your breathing. Ideally you want as small a rubber band as will return the linear gear to the starting position when you exhale. If you get enough generating capacity either through high gear ratio, extra motors, or a bigger motor, then you will need a bigger spring return. Essentially you are storing mechanical energy during your inhalation that is used to turn the generator on the exhalation so that you can generate on both push and pull. You need the diode bridge to successfully take advantage.



So I strapped on this monstrosity and hooked it up to my trusty data acquisition box from DataQ. Attached is the voltage plot output of the generator before step-up conversion to the 5V USB. Basically the battery runs the step up converter and the breath generator charges the battery. In the plot you can see the leveling effect of the battery, with the voltage spikes when I was breathing. Actually I was approaching hyperventilation, but in the name of science. The results can be seen in the photo of the phone charging. One thing to mention is that I had to modify a USB cable to get the RAZR to charge as detailed on this website. I don't have any solid numbers on the power I was generating, I haven't come up with a good way to measure that yet.



Typical resting metabolism is on the order of 50-75W of which a substantial portion is due to breathing effort (I have seen north of 50%). So if we assume 25W continuous energy used for breathing, it seems reasonable that we could increase that 4% to harvest 1W for charging a cell phone. Based on my cell phone, and these assumptions it would take about 3 hours to charge the 3.7V 800mAh battery. Assuming 100% efficiency.



Sadly, based on the few measurements I was able to make, the breathing generator I built is putting out more like 50mW. Way to breathe no breath. It would charge the phone, but the NiMH battery would be doing most of the work until it was drained. Then you would have to breath for a day or so to recharge the NiMH battery. You were planning on doing it anyway right? So there is room for improvement. One area I am looking into is using carbon nanotubes and polyurethane to make an electroactive polymer generator. This is the type of technology that is being used to make boot-strike generators for the military.



Future improvements could get this device into the 1W range. Specifically, using a better DC motor (higher voltage per rev) and custom building the drivetrain to be more comfortable and better coupling to breathing motion.



I have been working on these types of devices in my kitchen/workshop for a while and would like to make this all public so others can get on board. Feel free to contact me with questions or for discussion. As the bard said, "and the domestication of the dog continued unabated."

