I have a husquvarna riding mower that has a fully charged battery, but it drags and won't start. What's wrong?

Hi,
It could be but the info is a bit lacking.
Many things can make it drag; it is called parasitic load. You may wish to try this:
YouTube - Lawn Mower Repair Valve Adjustment (http://www.youtube.com/watch?v=NjzX-PUjKmo)
Peace,
Clarke