I have a Grasshopper zero turn mower with a Kohler 21 HP gas powered engine. The engine drains the battery when in use so I'm guessing the rectifier/voltage regulator is bad.. Not sure where the rectifier is, took a quick look today and couldn't find anything that looks like it. If there is any other reason the silly thing won't charge I'm at a loss as to what it could be. Battery charges up with a charger but after mowing for an hour when I turn it off it won't start again. The battery is new so it's not the problem, any ideas?

You guys helped me fix my Craftsman riding mower carb problem so I'm hoping someone knows something about my latest problem.
Thanks

 

Once battery is fully charged use a volt Ohm meter set on DC and check it at rest.
Should be about 12.6 volts.
Turn key a and turn on the lights a few min. to lower the charge.
Now start the mower and check the battery again. It should read between 13.5 and 15 volts if the chargers working.