@Job Van Dam: Instead you might try leaving static the part of the image where the text goes (a viewport just for displaying that text of course, would be displayed over a part of the background main image), and make the images which form the battery around the text change (in separate viewports). Not exactly want you wanted initially, but it could be a good approach... although a bit more complicated depending on your general design... it depends on how important the "% inside the battery" is for you I guess
Edit:
Putting it only in words is not so easy, so I made a little sketch. On the first image, the red lines define the viewports (central part is the % text viewport). You would use four %?bl tags, one within each viewport, all of them with exactly the same amount of conditionals inside. On the first power stages, only the %?bl tag placed in viewport 1 would contain the appropiate filling images for those stages. The %?bl tag conditionals for those stages in viewports 2,3 & 4 would be empty. In the middle power stages, the %?bl tags in viewports 2 & 3 would place the "battery filling" images, while in viewport 1 images for initial power stages would be loaded, and for viewport 4 conditionals would be empty. Similar logic for the final stages. In that way you would achieve displaying something like the 2nd and 3rd icons... using perhaps something more ellaborated under the % text (in my example simulating maybe a lable or a different battery inside look...) again, all this depending of how important it is for you to display this