The brief
The C180 was cranking sluggishly, the battery warning light was flickering at idle, and there was a faint whining from the alternator area. Three signs of the alternator at the end of its life, not just a tired battery.
The alternator is what keeps the battery charged and runs the car's electrics while the engine is on. It is spun by the accessory belt. When it starts to fail, it does not put out enough voltage, so the battery slowly runs down, which is the slow crank and the flickering warning, and worn bearings inside it whine. A car that is not charging properly will eventually leave you stranded with a flat battery.
The diagnosis
The scan named it directly: codes for an alternator mechanical malfunction and an open circuit on the charging connection, plus a low-power-supply complaint. A battery load test came back healthy, ruling out the battery as the cause, and the alternator's output measured below spec at idle and under accessory load.
The whining matched bearing wear inside the alternator, which is end-of-life. So the fix was a new alternator, and with the accessory belt and tensioner the same age, doing those at the same time made sense while the belt was already off.
The work
The negative terminal was disconnected, the accessory belt released, and the failed alternator removed. A new Mercedes-spec alternator went in, with a new belt over a new tensioner, the belt re-tensioned, and the wiring reconnected.
Then the charging output was checked on the test gauge across idle and load to confirm the system was holding voltage properly.
The outcome
A crisp half-second crank from cold, the battery warning light steady off, no whining at idle, and the charging voltage solid under load.
The C180 went home with the charging system reading clean. For the owner that is a car that starts every time and a battery that stays charged, plus a fresh belt and tensioner into the bargain, rather than a slow drain that ends with a jump-start in a carpark.