Although piezo elements generate plenty of voltage, they do not generate many amps. We can solve this issue by wiring the all the piezo elements in parallel (in other words, the positives are always soldered to each other and the negatives are always soldered to each other). In this way, we will be able to increase the amount of amps and consequently charge our devices faster. In order to prevent wires from braking off of the solder joints, use hot glue to fasten part of the wire in place so that the stress on the wires is not enacted upon the solder joint. You can connect the piezo elements from one side of the plastic to the other by threading the wires of one of the piezo elements through the holes so that they can be soldered in parallel to a piezo on the other side. Once all of the piezo elements on the heel piece are wired in parallel, solder the last piezo in the chain to one of the piezo elements on the toe piece, and continue to solder the piezos in parallel until all 14 elements are connected.