Apparently a cell had failed and the battery was not able to hold a full charge.
On shutdown the EFIS shows backup battery voltage. I noticed that during the 30 second shutdown cycle (during which the backup battery is powering the EFIS after master is switched off) that voltage was not 12 volts. During subsequent shutdowns the backup showed lower and lower voltage until it would not support the entire 30 second shutdown cycle.
Bottom line is, if you shutdown using the master, and the backup does not show 12 volts, the backup battery is probably failing. (Or possibly your system is not charging it properly)
RV4 N444JT (