what will cause a battery to overcharge and boil over on 2001 GMC Sierra 1500

I have replaced both my battery and my alternator on my truck thinking this would resolve the issue with the battery boiling over. the alternator was checked both with engine running at idle and at around 2000rpms. the alternator checks fine. what else could cause the battery to boil over?

Asked by for the 2001 GMC Sierra 1500
The battery is a reservoir for electrical energy the alternator replenishes current taken out of the battery once the energy taken out of the battery. Place a voltmeter across the battery it should read 12.6 volts at rest engine off, and 14.6 volts max with the engine running. If rest voltage is less than 12.6 volts charge and test the battery, if voltage across battery engine running is over 14.6 volts the voltage stabilizer is bad.