We’ve found that loggers with the Digi cell boards on them have a tendency to act a little squirrely sometimes and give either erroneous readings or cause excessive power draw. In you case, I’d start with putting a fully-charged 4400 mAh battery on your station and see how it performs for a few weeks. If that doesn’t solve it, investigate the solar panel, making sure the wires are okay, and putting a voltmeter on the panel output (with no load attached) on a sunny day. We’ve seen panels fail in the field but look physically fine, but they either don’t generate the proper voltage (6v) or enough current. If neither of those solutions fix the problem, then try replacing the cell board, or at least removing the cell board from the logger for a week or two and see what the data on the card says. A Mayfly logger without a cell board should last several 3-6 months without a solar panel with 5 minute sampling of a CTD. So seeing what a couple weeks of non-cellular logger activity looks like can give you a clue as to what the issue might be. But to start with, just put a different, fully-charged battery on and go from there.