hi there, battery is located under the seat.
http://www.modernvespa.com/pix/techlib/installtinytach/64622805-M.jpg
unscrew battery cover:
http://www.modernvespa.com/pix/techlib/installtinytach/64622928-M.jpg
battery!
http://www.modernvespa.com/pix/techlib/installtinytach/64623101-M.jpg
photos credits from http://www.modernvespa.com