Try to avoid this situation at all cost. After all, you must be able to show a valid ticket to the train attendant at any time.
If your battery is dead, the train conductor will make a request for regularization. You will not be troubled for the rest of your journey.
If you can prove that you had a valid ticket in the 4411 app within 14 calendar days (including the date of the check), your file will be put in order.
More information can be found on the NMBS website (www.nmbs.be).