Mystery resolved. I had to unscrew all bolts holding the tank and lift it a few centimeters. Sufficient to reach the battery terminals (I plan to install cigarette lighter socket)

http://www.photoshack.com/displayimage.php?pid=7666
http://www.photoshack.com/displayimage.php?pid=7668
http://www.photoshack.com/displayimage.php?pid=7667
http://www.photoshack.com/displayimage.php?pid=7669