Charging system overvoltage

The charging system is delivering more voltage than it should, so everything connected to it is running above its designed supply.

This is one of the few electrical faults that damages while you wait. A low supply makes modules misbehave; a high one degrades them. Electrolytic capacitors, lamp filaments and semiconductor supplies all age faster above their rated voltage, and a battery held above its charging limit gases, loses electrolyte and — on lithium chemistry — heads toward the failure that does not stop when the current does. So it is treated as urgent rather than as a reading to keep an eye on.

The signature is that the complaints have nothing to do with each other. Because the supply is shared, symptoms appear across systems at once: bulbs failing repeatedly, a battery that needs topping up or has swollen, modules resetting, several unrelated codes arriving in the same week. Codes naming a shared supply are the corpus's oldest argument in its most literal form — one thing wrong upstream producing a list of healthy parts.

What is actually at fault depends on the vehicle, and the alternator is not the only candidate. On a conventional car the regulator inside the alternator is the usual answer, but a poor earth or a broken sense wire can make a perfectly good alternator overcharge, because it raises the voltage until the point it is measuring looks correct. On an electrified vehicle the low-voltage system is fed by the DC/DC converter rather than by an alternator at all, so the same symptom points at completely different hardware. Establish which supplies the 12-volt system before anything is replaced.

Measure at the battery with the engine running and load applied. A figure comfortably above the normal charging window confirms it in seconds, and it is one of the few faults an ordinary multimeter settles outright.