Yes—on the EOS 5, very long exposures do rely on battery power, so the battery can limit how long the shutter stays open. Based on user experience shared here, the practical maximum is about 6 hours before the battery runs down and the shutter closes. In cold conditions, battery performance will be worse, so expect shorter times for night or winter shooting. Because the camera needs power to keep the exposure going, removing the battery during the exposure would not be a good idea.