On a few occasions, after my car has sat for a day or two, and there was rain fall during that time, the battery icon on the dash lights up. Battery voltage is 11.4V when this happens and the cables are snug on the battery posts. Nothing looks loose around the large positive connector, and there is no buildup around the terminals. I measured the voltage drop from battery post to the battery clamps and down a bit near the battery, I saw minimal drops (less than 0.020v).

I read codes when this happened, there was a B2562 (confirmed - low voltage).

I was thinking it may be a loose connections somewhere, or water shorting something until the engine warms up and the water evaporates.

Any advice on where to look or what this could be? I was thinking of following the negative to ground. Then following the two wires from the positive. Finally checking if any dust boots/covers are missing or lose on the positive cables.

edit: car is 2008 G37s coupe 6MT

 

Disconnect the battery and check the connections at the alternator. There have been reports of fluids (oil, power steering) slowly dripping onto the alternator which, eventually, shorts it out.

However, at that age and mileage, it would not hurt to remove the alternator and have it bench tested- not the easiest task to remove but doable.

 

a battery light on means an issue with the alternator

 

All the connections looked good. No rust, no heavy oils on them.

Dropped car off with the mechanic who said it is indeed the alternator. Though I am not sure what diagnostics he did. It is being replaced.

 

literally what I said, you could have said yourself the money

 

A qualified mechanic or repair shop can bench/ load test the alternator to determine its overall "health." They have tools and equipment that can test the diodes, the rectifier bridge, and, in some models, the voltage regulator. They can also test if the bearings are in good condition as well. Just because the "external connections" looked good, there is no telling the condition of the "internal components."

If I were to guess in OP's scenario, dampness from the rain got the brushes wet. Water, as a conductor of electricity, caused the current to arc, or short, to the alternator body. As the water evaporates the current flows normally. This would explain the intermittent behaviour of the "idiot light."

Additionally, dirt, grime, and oil (power steering, motor oil) leaks can seep into the alternator housing potentially causing issues as well. There have been reports here on the forum of this very issue. Also, due to age, it is possible that the alternator just "wore out." After all, the car is a 2008 so the alternator is at least 16-years-old

Whatever the cause, it is a rather easy and inexpensive repair.