I have 3 kittens who will be ready to test the outdoors in the next few weeks. I would really like to have trackers on them. If you use a tracker, which one did you go for and why? They are quite slight cats so I'm looking for something not too chunky. Do you have to charge it or does it run on a battery that means replacing every so often? There's so many options and costs, give me all your pro's and cons!