incase anyone cares, this is the response i got from eagletree:
The eLogger V3 will record down to 4.35V when being powered by a battery backup.
When used with the eLogger, the CAB-BAT-BACK can supply additional power from a battery ranging between 4.5V to 6.5V. Anything above 6.5V has the potential of damaging the eLogger, so we would recommend against using anything that is above 6.5V.