What is the little box that I see connected to the battery of my Jaguar?

I have a Jaguar Xj6 I believe that the battery is toast! I need to get the battery out so I can take it to go get tested. while taking a look at the battery I see a box connected to the ground side cable?! I am wondering what this is? and if it is okay to remove the battery from my car without doing any further damage?|||why not just get it jumped drive to your local parts store and have them test in while its still in the car...almost every parts store has this ability...and in general anything attached to a cable is either a fuse or a ground splice pack...