The screen will have a battery with a cross flashing on and off all the time. Or if you don't get that the picture will just go.
Just change the battery when you need to and click download. You don't lose anything. I didn't anyway. I don't think you lose anything as long as you change it in time. Which you should as it should come up with the flashing battery thing.
There was another post on here about that. If the flashing battery warning comes up, just reset and download and it might work ok for longer. With mine the battery still kept coming up so I knew I had to change it.