Alternator overcharging ?

[Joker2]

Got a new Optima Blue top battery this year went out and noticed when going 2000 or more Rpm's my voltmeter ( new ) goes up to 17 / 18 volts. Checked it with a known good meter at the battery while underway. sure enough it read 17.5 V. Didn't have this problem last year with old battery, I didn't change anything as far as electronics go. How bad is this and how do I fix it, or at least troubleshoot the problem ? Thanks