ZEN Programmable Relays: Is the Battery Unit absolutely necessary?

No, the Battery Unit is needed to use calendar and clock functions like the weekly and calendar timers. It is also needed to store holding bits and holding timer/counter present values when the power is turned OFF. Use the Battery Unit if the power will be OFF for 2 days or more at 25 degrees C.
The ZEN calendar and clock are backed up by the capacitor when the power is turned OFF, but may be lost if the power is OFF for 2 or more days at 25 degrees C. If the power will be OFF for 2 or more days, install a ZEN-BAT01 Battery Unit. The Battery Unit has a service life of 10 years. (If the Unit is completely discharged, the clock will automatically reset to January 1, 2000, 00:00.)