Just as a heads up, AFAIK there is no solar panel that is “approved” for use with the XT cameras. There is a community member that designed one for use with blink cameras and is selling it on amazon. Not knocking that particular solution, just clarifying that it’s not developed by blink and wouldn’t necessarily be something that customer service/tech support can help you with.
Having said that, if the solar panel is supplying sufficient current to the camera, it shouldn’t matter whether the batteries are in it or not. If the solar panel is unable to supply adequate power to the camera because of cloud cover etc. (and the panel’s internal battery bank is not charged) then the camera will shut off unless there are batteries in it.