Battery failure

[Themoog]

The answer to this may be fairly obvious but here goes…
Circa 6 months ago I was sitting in the X5 4.4 with the radio on for 10 min or so and when I went to start up the battery was flat. Got the wife to bring me a spare from our Range Rover that I had in the garage so I put that in and drove home.
Put the ctek on it.
All very good. Been on the car since.
Regularly have the ctek on as I only usually do quite short journeys.
Yesterday the same thing happened. Again 10 min with the radio on and battery was not powerful enough to turn the starter.
This time I’ve put that other spare from our motorhome in.
Anyway, all 3 batteries have been fully charged up with the ctek and I’ve tested them all with my battery tester and it shows that they are ok.
Im going to buy a new one today but just thought I’d run this by you lot for something to think about.
More often than not the car has a bit of a long crank which obviously is not good for any battery but also I have noticed a sound in the dashboard kind of like a fan or motor running when the door is open but the key not in.
Is it possible that I have a faulty heater resistor that is allowing the fan to run slowly sometimes?
Is there a way to check it? My climate system works perfectly as far as I am aware.
Thoughts?