Over voltage.... or not?

Hi buddies

I achieved the Madd Electrical modification (including a voltmeter instead of the ammeter) and my Demon runs very well

But...
My voltmeter reads 14 volts at idle and 15 volts while driving.
Is it normal or an over voltage?
Maybe my voltmeter isn't very accurate, I don't know.
Is a 15 volts charge dangerous for my battery? It's brand new.

François