Well from what I can tell the Flash card itself doesnt need any kind of batteries. THe linker does however. A lot of people just use a 9v Batterry since it works better however. Read the complete faq on www.gbaemu.com and you can learn how it works. Its very nice.
that is what i believed about the flash cards until i saw a picture of the 64, 128, and 256 MBit cards. The picture showed them opened and both sides with a watch size battery in each one.