[Al Skierkiewicz]

Jacob,
I am guessing you wish to avail yourself of the external power connector for this application. D is correct in that the external power supply is regulated. The internal power supply running off the battery is a separate power supply in most cases. It uses a form of boost/buck regulator to keep the various supplies at specification right down to the complete discharge of the battery. You could go to C cells and use 10 or 11 and a three terminal regulator for the 12 volt supply. A high current LM317 will do the job and supply the current but you will need to get rid of some heat on the regulator. Some of the sub C nicads would also do a nice job in this application. The drawback is the weight of the package. There are a variety of boost/buck regulators out there but cost and complexity do factor into the equation. As far as cost vs size/weight, it is almost better all around to find a cheap replacement battery for the internal battery.