The brief
The 320i came in with two jobs: the battery was getting on, slow to crank and a breakdown risk, and a tail light bulb had blown. No job's too small to do properly, and these two are quick wins worth sorting before the battery strands you or the missing tail light gets you pulled over. The battery starts the car and steadies the voltage for everything electronic, and on a BMW the new one has to be registered with IBS, the battery management system, so the charging system knows it's fresh and charges it right. A worn battery doesn't recover, so it needs replacing and coding in. The tail light bulb is a wear item that blows, and a missing tail light is a safety and a legal issue, so it just needs the correct bulb fitted and the lighting checked.
The diagnosis
A battery and charging test confirmed the battery was down, failing the load test and unable to hold voltage, the alternator charging fine; and a lighting check confirmed a blown tail light bulb with the holder and wiring fine. Two straightforward jobs. So it was a battery replacement, the correct type, fitted and registered with IBS, and a tail light bulb replacement, the correct spec.
The work
The old battery was removed and a new genuine BMW battery of the correct type and rating fitted, the terminals cleaned and the clamp torqued, then registered with IBS so the charging system recognised it as fresh, and the low-voltage faults cleared. The blown tail light bulb was swapped for the correct spec, the holder checked, and the full lighting set run through to confirm everything worked. A quick run confirmed a strong, clean crank, steady voltage, all the lights lit, and no warnings on the dash.
The outcome
A strong start on a fresh battery registered with IBS, all the tail lights lit, the rest of the lighting checked, and no warnings on the dash. The 320i went home sorted. A tired battery turns into a no-start and a missing tail light into a pull-over, so doing both properly, the battery coded in and the bulb the correct spec, headed off two small problems in one visit.