Uploaded file for and early concept for Version 2 of power stacker, which uses an isolated ground and a charge controller on each lithium ion cell for connecting in series without an external cell-balancer. It may be best to first target USB powered electronics and connect cells in series then use a switching regulator to step down the voltage. Connecting in parallel and attaching a diode to each pack does reduce the voltage of the pack slightly and every volt saved counts as extra minutes of run time!