At what voltage should batteries be replaced?
Depending on the total power demand of the system (number and types of connected sensors, logging interval, transmission interval etc.) the minimum acceptable operation voltage can vary.
At a minimum, however, it is recommended that the (16) D-Cell batteries inside an X2-SDL be replaced when the reported system voltage falls to 8V (1V per cell). Information on battery replacement can be referenced here.
NOTE: Below 7.2V, there is the potential that the logger may continuously reset and damage the data logger storage SD card in the process.