The brief
The Q3 had weak starts even though the battery had recently been replaced, with dimming lights, a dashboard warning, an unusual noise from the engine area, and electrical bits like the windows running slow. He brought it in. Weak starts with a fresh battery point straight at the alternator. The alternator keeps the battery topped up and runs the car's electrics once the engine's going, and when it stops charging properly, the car runs off the battery, so a brand-new one drains and the starts go weak. Dimming lights and slow windows are the same thing, the supply voltage sagging, and the noise is the alternator's bearings wearing. An alternator doing all of that has had its run, and it strands you once the battery's flat.
The diagnosis
On the multimeter the alternator wasn't delivering charge voltage with the engine running, where it should sit up around fourteen volts, and its bearings were noisy. The drive belt was tight, the harness checked clean, and the battery was the new one, fine, so the alternator's own regulator and bearings had failed. That's a replacement. A failing alternator only fails further, so it was getting changed.
The work
The drive belt was released, the alternator unbolted, and a new genuine Audi-spec alternator fitted, the belt re-tensioned, the connections cleaned. Then the new alternator was load-tested with the engine running and the major loads, headlights, blower, rear demist, switched on, to confirm it held charge voltage under demand. A road test confirmed the lights were steady, the windows quick, and the crank back to a clean half-second.
The outcome
Charging output back to spec, steady lights, the electrics behaving, no noise, and the crank back to normal. The Q3 went home with the electrical side solid again. A failing alternator strands you once the battery's flat, and on a car you've just put a new battery in it's worth tracing the real cause, so changing it put the charging system properly right.