The best thing I’ve found is simply switching the operation to disabled. This setting is saved through power outages and can only be changed back to enabled by the user.
While the operation is disabled the cost for keeping the sprinkler unit itself running is likely less than a dollar a year.
The internal battery is never used to power the device but rather ensure the saved settings persist through a power outage and should not be a source of concern.
Either disabling the operation or turning the controller off will both retain all your programs and settings so there will be no data loss or reprogramming. Furthermore, do note the UI has an export option which allows you to save your programs and settings.
Hope this helps!