The brief
The A3 had been slow to crank, the headlights dimming at idle, and a few electrical things acting up, the classic signs of a battery on its way out. He brought it in before it left him stranded, which is the right move, a dying battery only gives you so much warning. 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 cranking, the dimming lights, and odd behaviour from systems that don't like low voltage. On a modern Audi the new battery also has to be registered to the car's energy management so the charging system knows it's fresh and charges it correctly. A worn battery doesn't recover, so it needs replacing and coding in.
The diagnosis
A battery and charging test confirmed it, the battery was down, failing the load test and unable to hold voltage, which is what was behind the slow starts and the electrical niggles. The alternator was charging fine, it was just 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 look after it properly.
The work
The old battery was removed and a new 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, and the electrics behaving normally.
The outcome
Strong starts, steady voltage, bright lights, no electrical niggles, and the new battery registered so the charging system manages it. The A3 went home sorted. A worn battery only fails harder and drags the electrics down with it, so changing it and coding it in put the car right before the morning it wouldn't start.