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A replacement battery fitted without telling the vehicle about it. On anything with regulated charging this is a real step rather than dealer upselling: the module manages charging against a stored profile of the battery's type, capacity and age, and it deliberately charges an old battery differently from a new one.

So an unregistered replacement is managed as though it were the worn-out battery it replaced — typically undercharged, because the module is compensating for capacity loss that no longer exists. The new battery then sulphates and ages early, and the symptom is another flat battery a year later.

It also produces fault codes that look like hardware. State-of-charge performance, charging system performance and stop-start faults all follow from the module's expectations and the battery's behaviour diverging — with nothing physically wrong at all.

Type matters as much as registration. Stop-start vehicles need AGM or EFB batteries built for many shallow discharges; a conventional battery fitted to one works briefly and degrades fast. Registering an AGM as a conventional battery, or the reverse, is the same fault by another route.

Ask two questions of any recent battery change: was it the right type, and was it coded? Both are quick to check and cost nothing next to a third battery.