ok, ive seen alot of post lately about batteries. so here is the skinny on the battery for the 400ex..

stock part# YTX-9-BS CCA 120
this battery is manufactured by YUASA for honda is available by the aftermarket and your dealer.

the upgrade battery everyone asks about is:

STOCK PART # YTX-12-BS CCA 180
THE 60 CCA WILL MAKE A HUGE DIFFERENCE! in order to install this battery in the stock location you must remove(CUT) the top of the plastic housing of your battery box. also you will need to lenghten the stock holddown by 25 millimeters.

i hope this helps...