You need 1 CR2032 battery - https://amzn.to/3oIAdfB
2032 off eBay - http://ebay.us/vX6q9H

Here is a quick little video on how to change the battery on your S3, S4, S5, S6 and a bunch of other similar Audi key fobs! Hope this was helpful.

AUDI S LINE SMART KEY REMOTE GENUINE OEM FOB 4 BUTTON - 8W0.959.754.AN - FCC ID: 8W0959754AN

DIY How to replace your Smart Key Fob Battery for a Audi A3, A4, A5, S4, S5, S6 or Q7 that use the remote. This uses a CR2032 replacement battery. This is helpful especially if the battery is too weak to start the car.

A DIY battery replacement for the Audi Advanced Key Fob! No extra tools required! Currently for Audi A4, A4 allroad, S4, A5, S5, TT, TT RS, Q5, SQ5, Q7 - 2017 models or later with Audi Advanced Key. The replacement battery is an CR2032.
OEM AUDI S4 SMART KEY KEYLESS ENTRY PROXIMITY REMOTE IYZFBSB802 H71/0083 04S