When does it drop to 12.5? Honda's charging system allows the ECU to disable the alternator when it wants to. If the battery is fully charged & the ELD indicates no big power draw, the ECU will short the alternator "C" wire to ground. That tells the alternator to turn off. Then your voltage will go down to 12.5, battery voltage.

Wedley, I don't think 18v would be normal. Something's going on with your voltage regulator??