So, personally, I'm really excited about having wireless energy harvesting. I was looking up articles on piezoelectrics, since that seems to be the leading thing these days, and I encountered an article that designed a MEMS circuit (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3958224/). It's fairly recent, so I assume it's atleast close to modern piezoelectric tech. Basically, I'm wondering how long it would take for these systems to really charge a phone. The values seem quite small.. " 66.75 μW, or power density of 5.19 μW·mm−3·g−2 with an optimal resistive load of 220 kΩ from 5 m/s2 acceleration" But I'm not sure what it means in practical terms. How long would it take for a device like this to charge, say, a 3.6V smartphone with a 2000 mAh battery if it was operating at peak excitation?