The brief
This BMW came in needing a new battery, and on a modern BMW that isn't just a swap and away you go. The car keeps track of how old the battery is and tailors the charging to suit it, so a fresh battery has to be registered to the system or it won't be charged properly. BMW runs an intelligent battery sensor, the IBS, on the battery's earth lead, and it reports the battery's state to the car's power management. When the battery is replaced, the system has to be told it's a new one, by registering the new battery, so it adjusts the charging strategy to a fresh battery rather than an old, worn one. Skip that step and the car keeps charging it like the old one, which shortens the new battery's life and works the alternator harder than it needs. So the job is the battery and the coding, done together.
The diagnosis
A battery and charging test confirmed the old battery was worn out and due replacement, and the rest of the charging system, the alternator and the wiring, was sound. The car would need the new battery registered to its power management once fitted. That's a battery replacement plus an IBS registration and coding, rather than a bare swap that leaves the charging system working against the new battery.
The work
The old battery was removed and a new battery of the correct type and capacity fitted, the terminals cleaned and the clamps done up properly. The new battery was then registered to the car's power management with the diagnostic system, so the charging strategy is set for a fresh battery, and any related adaptations reset. The charging voltage was checked good with the engine running. A test confirmed the battery holding charge, the charging system behaving normally, and no battery or charging fault codes.
The outcome
A fresh battery, properly registered, the charging system tailored to it, no warning lights, and the electrics behaving as they should. The BMW went home with the battery sorted the right way. Registering the new battery is what gets the full life out of it and keeps the alternator from overworking, which is the difference between a battery job done properly and one that comes back early.