An actual 13.5 volts is not low for a charging system. 15 is high, in the automotive world. I don't know what the specs are for kohler, or if you have a problem with your charging system, just commenting about the #'s in general. EI, if I had a car that was putting out 15 volts, I'd be concerned about it overcharging and cooking the battery. The amperage is also important, as well you should use another voltmeter to confirm the one on the dash.
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