Usually – no battery backup. The reason is typically a weak or missing battery. In M91 models, as well as in all Vision controllers, if the battery is weak or missing, SB8 rises to 1. The controller has a special battery test loop. It lets SB8 rise at some time (usually 1-4 weeks) before the battery will become totally flat. If you find the battery flat and need to replace it, contact support@unitronics.com with the specific PLC model.