During the winter there is little light to power the solar panel and so the battery is used to power the node. The onboard LEDs continue to be powered even when the battery voltage is unable to sustain the node.
The LEDs will drain the battery beyond the safe voltage (due to the low but persistant current draw it'll work even though the LiPo has protective circuitry).
Solution:
Remove the LEDs
Use a 'zombie' mode on the node so conserve power when the voltage is low (this will allow the node to function through the winter with the occasional good solar charge).