The battery on the fk8 is a little weak and soon goes flat if the car is not used regularly.
Page 713 of the owners manual states that the battery should be disconnected before charging. However, if you do this, you will get the full Christmas tree warning light display on restarting the car on reconnection, and the ecu will spend the first few minutes of the next drive having to re-learn all the systems.
I couldn’t be bothered with that hassle on my 2018 fk8, so use a CTEK msx 5.0 battery maintainer without disconnecting. Been doing it regularly for 3 years now with no problems. Bear in mind that if you do charge without disconnecting the battery, that you do so at your own risk because of the requirement in the manual.
Worth noting that most dealers will connect a similar maintainer to your car whilst they work on it to avoid handing the cars back to customers with flat batterries.
If you do decide to charge without disconnecting, you would have to use a good quality battery maintainer such as the CTEK msx 5.0 though, not just any old charger.