I think only the newest device gets long battery hour (L-03E from docomo get rated for 13 hour, 10 hour via telecomsquare)

What I always do is to bring (or get) a portable battery / car charger and continuously charging them. Panasonic and Sony both does it and they are not that expensive (3000 - 7000 yen depends on power level). A good buy if you always having battery issue with your phone back at home