This started as way to possibly extend the battery life of my DeWalt crossline laser. The laser takes 3 AA batteries in series, for a total of 4.5 volts. I replaced two of the AA's with 14500 cells. In the third space, I put an LM7805 (to be replaced with a switching regulator when it arrives) inside of an AAA to AA adaptor shell. I soldered the output of the regulator to the positive of the dummy cell, the input to the negative, and a wire from the bottom to the negative of the tool that is attached to the ground of the regulator. Certainly not the cleanest, but It works, inspite of being a half volt above nominal. With no heatsink or airflow, the linear regulator gets pretty warm (33º C) after 10 mins of continuous use, so I'm looking forward to getting a mini switching regulator. This will increase the efficiency, and ultimately the runtime of the tool.
AAA to AA battery adaptor
It's basically a plastic shell that allows you to use an AAA battery in place of an AA in a pinch
Power Management ICs / Linear Voltage Regulators and LDOs
14500 lithium cells
3.7v Lithium cells that are the same size as AA batteries