I had my CLS55 permanently on charge with the CTEK living in the boot and the mains cable trailing out through the boot seal. To connect to the mains I had a 20m extension cable fitted with a weatherproof 13amp socket, the CTEK plugged in and the socket dangled just off the ground behind the car. All through the foulest of weathers, no issues at all, just a fully charged battery and a happy car.