I saw that the other day....it seems many, at least, are made of what appear to be 4 double-length D cells (which may or may not be 2 D, possibly C, cells wrapped together).
http://i195.photobucket.com/albums/z...etchum/6v3.jpg
and similar to this idea
http://www.jmsonline.net/HUMMINBIRD-...ABLE-UNITS.htm
Many batteries are made up of smaller cells, the questions are if it is safe to open them and if the smaller cells are really equivalent to their individually labeled counterparts. Try and your own risk (your health and your electronics may be at stake)