The documentation for the SED-THSU5045 thermostat states that the super capacitor voltage can be measured across terminal strip TB1 terminals 1 and 2. However, there is no mention of what an acceptable voltage reading would be.
Super capacitor voltage specification is not published on the thermostat documentation.
A minimum voltage of 5.5 VDC should be present across TB1 terminals 1 & 2.
If a 5.5 Volts DC charge cannot be maintained on the super capacitor, the device may require either an auxiliary light collector or external 24 VAC. The Thermistat battery is not designed to run the thermostat for extended periods.