An examination of the battery did not reveal any issues, so we proceeded to take down our red-neck router enclosure and further trouble-shoot the issue.
It turned out that we had an ant infestation... tiny red biting ants, to be specific.
I did not seal the small hole that the power cable passed through, because I thought that any moisture which condensed should have a way to exit. I did not guess that tiny ants would climb the power cable to the top of the tower to live in the cooler.
Note in the pics below how they have tunneled through the polystyrene and foam cooler insulation!
