I have a jd 318 and my battery light is always on now and when running it drains the battery pretty fast. What would cause this?

Sounds like your alternator is not charging the battery.

Check the obvious first. Make sure all connections to the battery are clean and tight, if you have an inline fuse check and make sure its good.

If all this checks out, then you may have to troubleshoot your charging system.

Best of Luck...

Is the alternator part of the starter like the old kohler ones?

Your tractor shows to have an Onan engine. The alternator shows to be under the flywheel with an external voltage regulaor.