Where I work we often have need for a dremel tool, and the one we have at our store has always been pretty weak. Rebuilding it's battery (which is made of 4 NiMH AA cells did help some but it was still really weak when you went into anything strong. I was going to hook up a similar battery while charging the normal one but when I opened the casing to solder on another battery temporarily I noticed the motor was rated for 7.2v, so Instead of temporarily hooking up another 4.8v pack I hooked up a small 6v lead acid battery. Easy to charge and much more run time. You could hook it up to a 7.2v battery too if you like, but I find 6v makes the whole thing much more powerful and doesn't add any more heat while running.
Essentially all I did was replace the rotary contact for the power switch with wires soldered to the motor contacts and a switch hot glued to the back of the dremel where the battery used to be, then the long wires fed to either a good 6v wall wart or 6v battery such as this: http://www.batteriesplus.com/product_search/32650-Werker-6V-5AH-AGM-Battery-W_--dot187-Terminal-DASH--DASH-WKA6--5F.aspx