Where the battery lives depends on the size of the Audi. Compact models such as the A3 and Q3 have it under the bonnet. From the A4 upwards it is usually under the boot floor, beside or beneath the spare wheel well. The Q7 is the odd one, with the battery under a front seat on many versions. Wherever it is, a start-stop Audi was built with an EFB or AGM battery and must get the same technology back.
Audi's energy management keeps a record of the fitted battery: its capacity, technology and a serial code. After replacement those details are entered with a diagnostic tool, a step usually called battery adaptation or coding. Without it the car charges the new battery by the old one's history, and start-stop or comfort features may stay switched off. Recent larger models add a 48V mild-hybrid battery, which is separate and not replaced here. A memory saver protects settings during the swap.