You sure your volt meter is accurate?

 

Easy check to confirm voltmeter is to check a second car battery voltage with car on and off.

 

I'd call the shop that rebuilt the alternator - "something" isn't right here. (Shorted diodes in the alternator, would be my guess.)

 

that seems like quite a coincidence that the old alternator would be charging to 18-19 volts, then a totally different one that they had already rebuilt (because the old one was so corroded the internal bolts were breaking and couldn't be rebuilt) is also charging at 18-19v? This is a very reputable local shop that has been here for like 50 years or more.

 

Why did you check the first alternator for output voltage? Was there some other problem or did you just happen to check it after replacing the battery?

 

yes, we checked the voltage after replacing the battery and it was 18-19v when charging, normal when the engine was off.

 

If your meter read around 12.5 V before starting the car, then I would be tempted to trust your meter.
That is very odd, with two different alternators, voltage is 18-19V.
Can you test another car while its running to see what your meter reads? If it reads around 14V (as it should) I would take your alternator back and have it bench tested and bring your meter along!

18-19V is not very good for long term battery health.

 

Check the battery cables and clamps...