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The Handy Board’s internal battery is rated for 9.6 volts; this is generally adequate for running motors rated between 6 and 12 volts. However, some 6 volt motors aren’t happy with the extra voltage, and some 12 volt motors will run too slowly.
Here are instructions for modifying the Handy Board to accept an external motor battery that can be any voltage from about 6 volts to 36 volts.
Lyle Hazelwood implemented a high-current H-bridge circuit described by Chuck McManis and tested by Jeffrey Keyzer, and posted the schematic and some circuit notes.
Matt Harlan created a printed circuit board with a high-powered H-bridge motor circuit for the Handy Board. Documentation consists of the readme file, the 
printed circuit board artwork, and the parts placement diagram.