The batteries were just on the old ones (they were about the size and shape of garage door openers and had a light or something that would indicate when your toll was paid). The newer model of suction cup Sunpass does not have batteries but I don't believe they have any visual cues either.
Actually, it sounds like they got rid of all the battery ones about 3 years ago (they don't work with the system anymore). You can see pictures here:
http://realtime.blog.palmbeachpost....-sunpass-transponders-wont-work-after-dec-31/