Is the real time clock very important?
If the Pi has even a flaky internet connection it will get regular time updates via NTP to keep it’s clock accurate.
One user reported his Pi drifting forward 12 seconds per day without NTP. Could be worse depending on temperature, etc. Let’s say it gains 30 seconds per day, that’s still only 5 minutes every 10 days. I wouldn’t care if my watering drifted that much or more, others may. And I doubt my internet connection would be down that long.
But what if the Pi’s ethernet/WiFi konks out? I may not notice that. An RTC, like the one on the OpenSprinkler board, would help here. Since I’m using relays I better rig up a ping test to alert me when the sprinkler Pi can’t be reached.
Other thoughts / comments?