I have the same issue. I have my LEC stations (GMI_ECxx) running with 2G cell service and I did have one station that the solar charge chip failed and caused battery to drop below the 3.55 volt threshold for upload. However GMI_EC5 seems to be getting a charge during the day but is still not keeping up. I attached a screen from MMW TSA and you can see the drop-off in voltage on this sensor station vs the others. I also noticed a general drop off which I presume is amount of daylight. I am using the 2.5 AH batteries. I plan to check current draw to see if it is abnormal or caused by poor cell service.