I want to shoot a wedding with a SLR, and I am worried that the battery is not enough. I have seen video screens with external fill lights. I don't know what kind of fill light you are talking about. Generally speaking, the battery of the fill light (flat panel of led) is independent, not connected to the SLR. Just buy two more batteries.

If you are a flash, the standard hot shoe flash battery is also independent, and the contacts on the hot shoe are responsible for flash and data exchange. The external battery box can make the lamp last longer and call back faster.

Wedding photography is a major event in life. I suggest that it is right to hire professional photographers and videographers outside. As a family member or friend, when something bothers you, can you take two photos by yourself? Walking with the audience is not what you want to do.