Another camcorder question?

Question

There's two type of batteries on the camcorder. One of them is a big one and that's the rechargable battery. There's also a small round one. What's that one called and what's it used for? Wouldn't the rechargable battery be the only one needed to operate the camera?

Answer 1

The smaller battery is used to maintain the camera's clock so the video can be timestamped.