depends on exactly which speed600 you have. speed600 is a pretty generic term, there are literally hundreds of motors that are called that. Mabuchi makes dozens, if not hundreds, of motors that physical size, with different windings, so does johnson and sagami. Most of the motors sold in the hobby industry as "speed 600's" will take 10 cells fine as long as you prop/gear them to draw about 25 amps maximum.