Battery's can act strange sometimes.
I remember a couple of years ago, my car wouldn't start again if I stopped the engine for just a couple of minutes and left the headlights on.
I replaced the battery to no avail, so I suspected the alternator not doing his job. I installed a new one, but I still had the same problem.
In the end it was the battery, which showed 14V, but if I hooked up a bulb, the voltage dropped in seconds.
I changed it for another one, problem solved.
Just checked the charge across the battery terminals and getting 14+ volts. Spoke/email Mike at Dynamo regulaters and he agreed the battery is stuffed prob too many charge/discharge cycles. Also when I tested using the vreg it was only putting 12.8 volts into the battery so look like they both decicded to die which begs the question who killed who 
When reading your last reply though, it came to me that it's perfectly possible that the alternator in my car killed the battery.
I remember changing the battery first because of the same reason, then the alternator, and then had to swap the battery again for a new one
