This is one of those intermittent issues driving mad any car driver:
What I have:
New battery, new alternator brushes, new current regulator, new belt. Polished brushes tracks. All properly installed.
Problem is: sometimes the battery gets discharged in a 20 to 40 minutes evening trip. Not completely out, but not enough to crank the starter. This new battery comes with the magic eye, green means good, black means discharged. When this happens it obviously turns to black.
The car came with an amp meter in the dashboard. In any case, it shows good readings when the engine is running. When the engine is in idle it is a bit off the 0 +, when accelerating the needle moves towards the end of the scale if I gas it enough. During driving, the amp meter always shows some + reading. I guess, the alternator is dishing some juice, right?
But even under that circumstances, sometimes the battery gets discharged. In those cases, I can see lights dim on idle, getting brighter when gassing the engine.
So...what to blame?
Car is a 1970's Fiat 126p...no electronics, no computers, no fancy stuff, carburated, alternator with external regulator, regular headlights 90/50, no stereo. Just a bareback pony :)