Jason Vierik wrote in a message to Jasen Betts:

 >  GM> But a voltmeter that reads 12 volts... It's a 12 volt battery, so
 >  GM> everything must be alright, mustn't it?
 > 
 > When the altenator's running it should read closer to 14...

 JV> Yeah, doesn't an altenator run at or around 13.8?

13.6-13.8 is considered "float" voltage for a charged battery, 
you could consider that a reasonable minimum.  When I was selling batteries
we looked for 13.5-14.5,  with the engine running.  Some vehicles went even
higher,  though 16 volts was considered about the maximum acceptable
figure.  Any higher than that and there were regulator problems, 
especially if the battery was already charged at that point.

And regulators _were_ a problem all right.  I saw some,  especially those
found in Ford products,  that would leak current to ground when the car was
turned off.  Some would go dead short.  One guy with a brand new battery in
the car was irate the next day and had his car towed in to the garage
because that new battery was stone cold dead.  The garage brought it over
to me,  and the terminal voltage was darn near zero!  I did charge it back
up,  and it recovered okay,  the culprit in his case turning out to be the
regulator...

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