There is a safety block in the software that does not allow stitching with lower than 5% battery charge to avoid system crash in the middle of quilting when the battery fully depletes. When the battery is taken out, the tablet runs on AC-power, so this block is lifted. However, one should use caution when stitching in this mode, for a brown- or blackout will interrupt the stitching AND the tablet will lose the progress information.
When the battery becomes un-chargeable, we recommend to take it out permanently and plug the AC-adapter in a battery back-up power supply (UPS). The following unit would be more than sufficient, for about $60: http://www.tripplite.com/en/products/model.cfm?txtModelID=3116