Citroen C8 baterry discharge

you need to first make sure no lights being left on in car ie in luggage compartment unplug any phone chargers when car locked make sure no cooling fans running for long periods if ok you need to have a parasitic battery drain test done this is done by connecting in series an ammeter between battery teminal and lead use neg terminal and after leaving car to shut down note reading then remove fuses until meter drops (cars have some drain )it will be a component fed off that fuse could even be alternator i would get a good auto electrician to do this for you