I have the same problem - it does not charge the battery. My old G100 had the same battery for 6 years. It's a dry cell, so there's no water to add. Suggestions?
With the mower turned off, unplug the single wire coming off the voltage regulator, and check the voltage. Should be battery voltage. If not then there is a problem between the voltage regulator wire and the battery. Unplug the double connector off the voltage regulator, and start the mower.. with the engine running at full RPM, connect the leads to the two wires in the double connector coming out from underneath the blower housing, voltage will be AC, and should be in the range of 28-32 V. If those two test come out OK, then replace the voltage regulator.
If the voltage is low from the stator (wires coming out from underneath the blower housing) then you will need to replace the stator. It is under the flywheel.