Symptoms
Slow cranking, a battery light that comes on under load, headlights that dim at idle and brighten when revved, or electronics that behave oddly before anything else fails.
Likely cause
Lead-acid batteries lose capacity with age and heat. Alternators fail at the rectifier diodes, the voltage regulator or the bearings; the drive belt and its tensioner wear on the same schedule.
How should it be checked?
Have the charging system tested rather than guessing. A load test on the battery and a voltage reading at idle and at speed separate the two, and both take minutes.
How can you check it yourself?
Start the car cold and listen to how fast it cranks, then look at the battery's date sticker. A battery older than five years on a car being sold is worth raising.
The question to ask the shop
What did the charging system test read — battery state of health, and charging voltage at idle and at 2,000 rpm?