The brief
Ms Samina's E250 CGI had the battery charging light lit up on the dash, so she brought it in. That light is the car warning you the charging system isn't keeping up, and if it's the alternator, the car runs off the battery alone until it's flat and won't restart, so it's worth sorting before you're stranded. The alternator is the belt-driven generator bolted to the engine that keeps the battery charged and powers the car while it's running. Like any part it wears, the brushes, the bearings, the internals, and eventually it can't put out enough charge. The battery light comes on, the battery slowly discharges, and electrical things start playing up. The first thing to do, though, is check the battery itself, so you're not replacing an alternator for a battery fault, and then if it's the alternator, replace it.
The diagnosis
A battery and charging test came first, the battery was good, holding charge fine. The alternator was the fault, its output low, not keeping the system charged, which is the battery light. The drive belt and the wiring were sound. That's an alternator replacement, with the battery confirmed good first, rather than guessing between the two.
The work
The drive belt was released, the old alternator unbolted from the engine block and a new genuine Mercedes-spec alternator fitted, the belt refitted and the tension set to spec, the connections checked clean and tight. The charging voltage was checked good with the engine running, the fault cleared, and the battery's state of charge confirmed. A road test confirmed the battery light stayed off, the charging voltage steady, and the electrics behaving normally.
The outcome
No battery charging light, the charging voltage steady, the battery holding charge, and the electrics behaving as they should. The E250 went home with the charging sorted. Checking the battery before replacing the alternator meant we fixed the actual fault rather than the wrong one, and a healthy charging system is what keeps you starting first time.