The brief
The A6 had been cranking slow first thing in the morning for about a week. The headlights had been dimming at red lights when the AC fan ramped up. And on a couple of mornings the engine had answered the key with a single click rather than a crank.
None of those is dramatic on its own. Together, and at three years on the original battery, they are the standard signs of a battery reaching the end of its life.
The click instead of a crank is the clearest one: the battery has just enough left to energise the starter solenoid, but not enough to actually turn the engine over.
The diagnosis
A battery load test confirmed it. The capacity was below spec, the voltage dropped under a cranking simulation further than the healthy band allows, and the recovery curve afterwards was too slow. The tester's printout said it plainly: internal resistance well up, and a recommendation to replace.
We checked the rest of the charging side too, because there is no point fitting a new battery if the alternator is the real problem. The alternator, the charging system and the harness all tested fine. The battery itself was the failed component.
The work
Disconnected and removed the failed battery, carefully, because a modern Audi needs proper handling during a battery swap: keeping the electronics fed so modules do not lose their settings, and observing the right disconnect order.
Fitted a new VAG-spec AGM battery, matched to the original on capacity and cold-cranking amps. Then registered it on the scanner, so the charging system knows there is a fresh battery and manages it on the right charge profile rather than continuing to charge it as if it were the old worn one.
Skip that step and the new battery gets cooked and dies early. It is the bit a quick swap usually misses.
The outcome
Normal half-second crank in the morning. No dimming at lights, even with the AC fan up. No clicks.
The A6 went home with the electrical system reliable again. For the owner, that means a car that starts every time, which is the whole point of a battery, plus the smart features that depend on a healthy battery and a correct charge profile all working as they should.
The new battery is set up to hold its full rated service life as long as the car is driven regularly enough to keep it topped up.