You need to make sure the battery is fully charged when you replace an alternator. If it isn't the alternator can get overworked by trying to maintain vehicle and charge the battery at the same time. You can kill an alternator. Doesn't happen every time. But it can. I work at O'Reilly Auto Parts, I see it enough. Often times when either a battery or alternator go out the other follows from trying to make up for the other one going bad.
And just cause the battery reads 12.6 volts doesn't mean it has the Cold Cranking Amps to turn an engine over. You need to test it with a load tester to be sure.
2005 ZX4 ST, Stealth CAI (so far)
Used to have, and will always regret selling, 1970 Mach 1.
"Everday is a good day, just try missing one."
"As soon as we're born we start dying, so you might as well have a good time."