SL55- convenience functions

[PGN6]

If the car hasn’t been driven for 10 days or so I get a message saying the convenience functions not available. After around 10 mins of driving they all come back to life. Is this a battery issue- should I have the car on a trickle charger? Has anyone installed a charging connection- and if so could you let me know details. Thanks

[PGN6]

Thanks- for the replies- is there a specific ctek you use? Sorry to appear a bit thick but how do you connect it and where ?

[PGN6]

Thanks have ordered one