Bongo Fury

On a couple of occasions even after a long run I have got back to the van to find battery completely flat. I bought the battery from Halfords just over a year ago. They tell me it tests OK. When I start the van the charge voltage across the battery is about 14.5V but sometimes there is a sound like a relay clicking in and out and charge voltage can get as high as 16.5V. Both fails have happened on very hot days. Problems with ambient temperatures? Alternator? Voltage regulator? (is there one?) Or is the battery knackered. I'd appreciate any thoughts.

16.5 volts at the terminals of the battery is NOT a good sign,anything above 14.5 means your generating a lot of Hydrogen and Oxygen, sounds like regulation of the alternator is failing, possibly heat related.