Problem
Despite previously available solar energy, the batteries are fully discharged.
Possible Cause
Low temperature prevents charging, rather than discharging, so the battery was never disconnected from the system. The battery continued to serve the load but could not be recharged, resulting in a fully discharged battery that was unable to safely accept available charge.
Solution
The battery space (not the batteries directly) may need to be heated. If the problem persists, automatic space heating or insulation should be considered.