The brief
The A180 had a tired battery, slow to crank with the lights dimming at idle. He brought it in before it stranded him, the right move. On a modern Mercedes there's a catch worth knowing: when you fit a new battery you have to register it to the car. An unregistered new battery won't charge correctly, the car's energy management keeps treating it as the old one, so even a fresh battery ends up undercharged. The battery starts the car and steadies the voltage for everything electronic while it runs. They wear out, the capacity drops, and once it can't hold a proper charge you get the slow starts and dimming lights. A worn battery doesn't recover, so it needs replacing, with the correct type and rating, and then registered to the car so the charging system manages it properly from the start.
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 the battery at the end of its life. So it was a battery replacement, the correct type and rating for the car, fitted and then registered to the energy management so the charging would recognise it as fresh, the step that an unregistered battery would miss.
The work
The old battery was removed and a new genuine Mercedes 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 knew it was fresh and would charge it correctly, and any stored low-voltage faults were cleared. A quick run confirmed a strong crank, steady voltage, bright lights, and the electrics behaving normally.
The outcome
Strong starts, steady voltage, bright lights, and the new battery registered so it charges the way it should. The A180 went home sorted. A worn battery only fails harder, and on a Mercedes a new one that isn't registered never charges right, so changing it and coding it in properly did the job at both ends.