Battery not getting enough charge

[september2016]

Folk, I drove my 1980 MGB for about 90 miles the other day and all of a sudden the car died. The battery was dead. I got a jump and got home. I charged the battery which was new. Then I tested it with my multimeter. 12.4; perfect. I purchased a new alternator and installed it. Then tested it and got 14.25 while running. Turned it off and she ready 12.4.  Again, all as it should be. then I drove to the gas station and back (5miles total) and checked my reading whilst running and it read 12.18. You can imagine my concern. Turned the car off and now she reads 12.27. I am starting to kill the battery again. Took the alternator off and took it to the local parts store, had it tested and the said it test good; its charging. So then I checked all connections, battery cables, ground, belt is tight, not slipping etc.  In fact I changed all four fuses for new fresh ones and also changed the Voltage Stabilizer which is mounted on the fire wall above the steering column. Still the same readings.
Has any body had this before? I have not owned the car long so I don't have a long history with this car and frankly I suspect the car was doing this the entire time, because that 90 mile ride was the longest ride I ever took. 
Folk, any ideas or what I should consider next?

[Art]

First thing is to get the obvious out of the way. 

You've done most of the basics, but here are a few more 'next step' items:

- Did you have  the battery tested under load? You could be faulting the system for a bad cell or other battery defect.

- Have you checked for a voltage drain in the car?  A cable grounding to the chassis under a chassis clamp, a live cable connector touching the chassis or circuit open and drawing constant power could play havoc with the charging system.  Lucas electrics are known to "leak" and can drain a battery on a standing car very quickly.  It's why marine-type battery disconnects are sold by all the major catalogs and installed by most owners.  That, or to use a battery tender that maintains the battery's charge on a standing car.

- In testing the alternator, did the shop ensure the electronic regulator (internal to the alternator) is functioning properly?  These have been known to cause issues and may not always be evident on a bench test.  The alternator works, but the regulator may not. They have prompted many owners to convert to US Delco or other alternators that have proven more reliable in this regard.  Several Lucas types were used and, though the alternators themselves look the same and are interchangeable as assemblies, the internal electronic regulators are not between the different ones and won't work correctly if mixed-up.

I hope this helps you.  Let me know how you make out.  I'd be interested to hear what you find.

Safety Fast!

Art