The brief
The CLA180 had a degraded battery, slow to crank with the lights dimming at idle. He brought it in, the right call, and on a modern Mercedes there's a bit more to a battery change than dropping any battery in. These cars use a maintenance-free AGM battery, a more advanced design than the old wet-cell type, better at handling the heavy electrical loads and the start-stop cycling. The new one has to be the correct AGM type and rating, and it has to be registered to the car's energy management so the charging system knows it's fresh and charges it correctly. A degraded battery doesn't recover, so it needs replacing and coding in, done by someone who knows the car.
The diagnosis
A battery and charging test confirmed it, the battery was down, failing the load test and unable to hold voltage. The alternator was charging fine, it was just the battery at the end of its life. So it was a battery replacement, the correct maintenance-free AGM type and rating for the car, fitted and then registered to the energy management so the charging would look after it properly.
The work
The old battery was removed and a new genuine Mercedes-spec AGM battery of the correct type and rating fitted, the terminals cleaned and the clamp torqued properly. The new battery was then registered to the car's energy management so the charging system recognised it as fresh, and any stored low-voltage faults were cleared. A quick run confirmed a strong crank, steady voltage, bright lights, start-stop working, and the electrics behaving normally.
The outcome
Strong starts, steady voltage, bright lights, start-stop working, and the new AGM battery registered so the charging system manages it. The CLA180 went home sorted. A degraded battery only fails harder, and on a Mercedes a new one that isn't the right type and isn't registered never works right, so changing it for the correct AGM and coding it in did the job properly.