The brief
The Q2 was getting harder to start on cool mornings. Cranking had grown slower over a few weeks, and the headlights dimmed a touch at idle when the engine was not holding revs up.
The owner also noticed the dashboard lights flickering whenever the rear demister came on, or when the seat heaters and the aircon pulled together.
None of those on their own is alarming. Three together is the classic giveaway of a battery near the end of its useful life.
The other tell was the start-stop system. It had quietly given up and stayed disabled. The car does that on purpose when the battery's reserve gets too low to guarantee a clean restart at the next junction.
The diagnosis
Battery tester told us most of what we needed. Cold-cranking amps were well below rated, voltage dropped further than spec on a simulated start pull, and the reserve capacity was reading low.
Next we checked the alternator, because there is no point swapping a battery if the charging system is the actual problem. Alternator output across idle and load came back solid and inside Audi's range, so the charging side was healthy.
That pinned the fault on the battery alone. After several years, its internal resistance had climbed past the point where the BMS will trust it to support all the modern electrical loads, the start-stop cycling, the modules that wake briefly through the night, the big infotainment standby draw, and so on.
Replacement with a like-for-like AGM unit was the right call. With a BMS re-register, so the car learns it has a fresh battery.
The work
Negative terminal off first. Always. That breaks the circuit cleanly and stops any short while the battery is being moved.
Lifted the failed battery out of its tray, then cleaned the terminal posts and the clamps. They had a thin film of corrosion at the contact faces, which is normal after a few years and worth wiping off rather than reusing.
The new battery is an Audi-spec AGM unit matched to the original on cold-cranking amps and reserve capacity. We seated it back in the tray, torqued the hold-down clamp, and reconnected the terminals.
Then comes the bit that often gets skipped at less careful shops: registering the new battery to the BMS through the scan tool. Without that, the car keeps using the old battery's aged charging curve on the new one, which overcharges it and shortens its life.
Last check was alternator output on the test gauge across idle and accessory load, just to confirm the whole charging side was happy with the new unit.
The outcome
Crisp half-second crank from cold the next morning, exactly the way a healthy modern engine should fire.
Headlights stayed steady at idle, and the dashboard lights stopped flickering when accessories came on. The rear demister, the seat heaters, the aircon, all could pull their full draw without anything else dimming.
Most telling, the start-stop system was active again and behaving normally at red lights. That is the BMS quietly saying it trusts the new battery's reserve.
For the owner, that means the Q2 is back to behaving like a new car would. The battery should now hold its full rated service life as long as the car is driven regularly enough to keep it charged.