1.) Never full field an alternator with the regulator still attached to the alternator. If it doesn't blow it up right away, you will severely damage it. On a Dodge type system doing this will blow your computers up.
On a Ford it takes less than 10 seconds to disconnect the plug from the regulator. So why not do it properly.
2.) Never full field past 18 volts, and never full field for more than a second at a time. If you full field an alternator, and give it enough rpm, the little test lead you are holding on to can produce enough volts and amps to kill you on the spot.
3.) No 12vt alternator normally spikes to 18volts under normal operating conditions. Doing so will boil the battery almost instantly. It will also damage everything you have operating on electricity in your truck. Don't believe me, ask the guys running 16vt systems how often they change their headlights.
4.) As for voltage readings in the system:
12volts: Means the alternator is not working or severely over worked. If it is working but load down so bad to put out 12volts, it's life span is about 20 minutes.
13volts: Means you are running the alternator way down, life expectancy cut by about 75%
13.8: About the minimum aceptable voltage in a charging system. alternator is working at near full capacity. Will have some effect on alternator's life span.
14.2-14.8: Alternator is working at less than 80% capacity, this is considered normal operating parameters.
14.8-15.2: Little extra voltage, for a little extra power. Also stock voltage set point on some GM applications.
15.3+ volts: Over charging, and battery is being boiled.
Also note charge rates for batteries:
Low antimony: (really cheap batteries) like a charge rate of 13.8 to 14.1
Hybird (one l/a, one calcium) (interstate and others) like a charge rate of 14 to 14.4
Calcium/calcium (east penn/delphi) can handle charge rates upto 15.2