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The alternator producing a voltage above what the system should see, usually through a failed regulator. Overcharging is more damaging than undercharging and far less often suspected — the battery still starts the vehicle, so nothing obvious is wrong until something fails.

High voltage boils the battery's electrolyte and shortens the life of every module on the vehicle. The signs are a battery that needs topping up, a case that is warm or swollen, and a smell of sulphur — plus bulbs failing more often than they should, which is a genuinely useful early clue.

Modern vehicles vary charging voltage deliberately, so a reading that looks high may be a correct part of a smart charging strategy. Compare against the manufacturer's specification rather than the traditional fourteen volts, and measure at the battery with the engine running under varying load. A voltage that stays high regardless of load or state of charge is a regulator that has stopped regulating.