On the subject of mixing supercapacitors with lead batteries, check out this report at GCC on a CSIRO initiative:

http://www.greencarcongress.com/2008/01/csiro-ultrabatt.html

You can also see the AFS Trinity VUE hybrid using supercapacitors to increase battery life. They have submitted a provisional patent on their design and are trying to get auto companies interested.

http://www.greencarcongress.com/2008/01/afs-trinity-unv.html