If power is drawn from the solar battery at a greater rate than the battery is being charged, the battery will eventually be depleted. The battery is able to maintain a charge overnight unless the station is contacted frequently. Station downloads and launches require more power than a query of current conditions by the real-time monitor.
Under normal lighting conditions, the solar battery should be able to make 48 real-time monitor calls per day or 8 downloads per day. Setting WebLog on a 15-minute interval will deplete the battery as will setting the real-time monitor for Repeat Refresh will also deplete the battery.
If the battery is not retaining it's charge with infrequent use, check that the solar panel is clean and that the connections are secure.