I heard the battery in the yfz puts out like 14 volts and needs high rpm to properly charge the battery. Easy trail riding during the daytime without your lights on will drain the battery, as the rpms aren't generally high enough to excite the stator, and your cooling fan cycling on/off adds to the problem. :angry
The recommended fix is to properly charge your battery FULLY with the Optimate charger/tender BEFORE you ride it for the first time, and before every ride. Riding with your headlights on excites the electrical system to charge the battery at lower rpm's. An Optimate charger retails for about 45 bucks at your local Yami dealer :rock