The two alternators may not have the same capacity, ie, one may be a 90 amp and the other a 120 amp which would explain the different charge rate. Also when checking a charged battery, you need to wait about a half hour to allow the surface charge to bleed off or turn on the headlites for 30 sec and then check.

 

Sorry for the confusion. The old battery would show a battery light and very slow to charge from alt on the guage. A few blocks before charged. The new battery has no battery light and take maybe 5 seconds to show charged. I know 5 seconds isn't much. But, the tahoe does it quick! Wondering why?

 

I just replaced the altenator in my 04 suburban and it takes a few seconds to hit 14.2 volts on the dash gauge from my understanding its normal
Prolly was just a faulty batt

 

Yesterday it no longer charged. Had local shop put a new alt in. Seems fine now. $150 for lifetime alt and $45 install fee. Thanks guys