Hi
I think you would need to speak to the manufacturers for an answer to that question.
I would say that a cable carrying low voltage (12V) is much more sensitive to length than one carrying high voltage. Going back to my school physics - Heat generated in the cable (and therefore power lost) is proportional to the square of the current, times the resistance. In a low voltage system current is much higher, as you increase the length of the cable you will increase the resistance. I don't suppose the cable will be carrying a lot of current it you are trickle charging, however it might be different if the battery is low.
I suggest you would be better off putting the ctek in the car and using a standard 240V power extension cable - cheaper too.
David