It really depends on how long you get breaks in mains power, if you can connect a 35HVS to your Clock Controller and it displays the correct date, then it will do this indefinitely because the unit will be powered 24/7, the only situation that the backup battery will be called into action will be during a power cut, so if you only get a power cut for a maximum of say 2 days, then the battery backup only has to last that long
I'm not sure where the 35HVS is placed, is it next to the clock or is it on the main building?, I guess it needs to be waterproof?, I am asking because it would a lot easier to install if you didn't have to remove and reinstall the cable to the 35HVS, if it had a plug and socket arrangement very close the 35HVS itself if would take only a few seconds to remove / install / re-install which would help with testing the unit(s)
