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The belt driving the alternator and other accessories, worn, glazed or insufficiently tensioned so it no longer turns them reliably under load. The alternator is where it shows first, because charging is the most electrically visible thing the belt does — so the codes name system voltage while the fault is mechanical.

A belt does not need to break to cause this. A glazed or worn one grips well enough at idle and slips when the alternator is loaded — headlights, heated screen, cooling fans on a hot day — so the voltage dips exactly when demand is highest. Charging faults that appear only under electrical load, with a healthy alternator and battery, are worth taking to the belt.

Check the tensioner as well as the belt. An automatic tensioner whose damping has failed lets the belt flutter under load even at correct static tension, which is invisible with the engine off. Look at the belt's ribs for glazing, cracking and missing material, and watch the tensioner arm with the engine running — an arm oscillating visibly is the fault, and a new belt on a failed tensioner will slip too.