The brief
The 318i was due a new battery, the old one showing its age, slower cranking and not holding charge the way it should. He brought it in, and the right approach on a BMW is more than just dropping any battery in. A BMW original battery is built to the car's specific requirements, which matters because the electrical systems on these are sensitive to battery health. And after fitting, the car needs the new battery registered with Intelligent Battery Sensor coding, which synchronises it with the vehicle's management system so the charging strategy adapts to a fresh battery rather than treating it like the old worn one. Skip that step and the new battery doesn't get charged properly and other systems can play up, so it's part of the job.
The diagnosis
A load test confirmed the old battery was at the end of its life, its state of charge and its ability to hold voltage under load both below spec. The alternator checked out, charging properly, so it was just an old battery, not a charging fault. That's a replacement, with the new one coded in, rather than chancing it on a tired battery.
The work
The old battery was disconnected and removed, and a genuine BMW battery of the correct type and capacity fitted, terminals cleaned and tightened. Then the new battery was registered with IBS coding so the charging system recognised it as fresh, and the crank and voltage checked to confirm a clean start and steady supply. A road test confirmed the electrics were rock-steady and everything was behaving.
The outcome
A clean, quick crank, steady voltage at idle, the electrics behaving, and the new battery coded in so the charging system manages it correctly. The 318i went home on a firm electrical footing. A tired battery is one of those things that's easy to live with until the morning it doesn't start, so swapping it for a genuine one and coding it in did the job properly.