I had the same problem with my 07 yfz450 and i changed it out with a 400ex battery from yuasa,BUT my charging system didn't have a problem with charging. In fact it boiled the acid out of the batterys. the guys at the shop said to run the fan constant with a toggle switch and turn the lights on high. the battery lasted three rides and boiled out. I guess now i get to buy a stator and regulator to see what that does