Something I learned last night - my nephew's car stopped on him recently. When we got it home, it managed to fire up, but only had 11.5V at the battery. Immediately suspected pooped alternator.
Anyway, went to fit the replacement alternator last night, noticed the belt seemed a bit loose, and the -ve battery terminal was loose. Tightened them both up - va-voom 13.5V with headlights on.
So, the point of this story - is the fan belt tensioned correctly, and the battery terminals done up tight?