might consider using a BR battery then opposed to a CR; my concern with the battery is temperature range… a winter out in a greenhouse (or even a detached garage) in colorado will likely drop down below the -4F thats the lower limit of the battery.. once they sit at -20 for a week or so they tend to perform poorly afterwords then die prematurely.
The BR is rated to -22F which is more reasonable (although Ive seen it get lower than that outside here); I switched to supercaps for RTC’s when making circuit for a high altitude balloon… all sorts of battery issues @ 120k altitude & -40 degrees.. We also use em for the GPS in OpenPilot, where thousands of dollars in damages and possibly injury can happen if GPS lock is lost from a brownout… They take up less space on the circuit board, are lighter and more rugged (no loose battery after a plane/balloon crash).. as a result of many great successes with supercaps its my go-to over an onboard battery.
The problem I see is when the battery dies nobody will notice until after there is a failure; depending on scenario a small power outage in middle of heat wave (when the electrical grid is the least reliable) might destroy a crop if the timers dont come back when the power does; or land you a ticket from local govt for not following irrigation restrictions.
I dont know how old my analogue sprinkler controller was; I suspect over 20 years… and it ate up batteries in my garage. I’d like to think my OpenSprinkler will be in service for the next 20 years, but changing the battery in it is even more of a pain than in my old controller since I’d basically have to unmount and disassemble the entire thing.
In my tests with a very high frequency/accurate crystal & RTC with the biggest supercap digikey offers; I was able to maintain time for at a minimum of 10 days without power, it was really close to 2 weeks total; which should more than cover any reasonable unplanned power outage.. If your without electricity that long I dont think your sprinklers are much of a priority; you have bigger issues.. Yes people whom unplug there controller over the winter will have to restore time in the spring but they also wont have any reason to question the reliability of the battery after a prolonged usage in extreme temperatures,. I figure since manual action is required to plug it back in there is obviously someone around to verify that internet is online and the time got set correctly.
And now, adding a ground-plane with unused copper on the pcb, connecting the pcb ground to that and providing a header for access.. My Transformer has a ground connection, my old 20+ year old controller had a ground connection, if you make it available people will use it. I have a lot of expensive radios and computer equipment in my server room and as good policy I dont allow anything connected to an outside wire to enter my room that hasent been grounded before it enters my room. I will install a ethernet surge protector with a STP ethernet cable connected to my OSBo and and ground it to the transformer ground; this meets my requirements, and should also provide a proper ground to the Beagle; that rain sensor mounted to my gutter, connected to the beagle, wired to my server room makes me nervous.. Come to think of it; regardless of having a ground on the OSpi its best to install an ethernet surge device if your using wired ethernet, but my next point:
Right now the USB port on the microcontroller is floating ground, I suspect if this was grounded properlly WiFi USB may be less susceptible to noise from the AC coil and power running nearby.. I know laptops with usb radios have all sorts of issues until you plug it into the wall and give it a ground.. You can run a USB Spectrum analyzer or USB-SDR and see an improvement just by plugging a laptop into mains vs running off battery w/floating ground (I am also a HAM with a bit of experience).. I bet if you had a HID ballast nearby (Common in indoor gardens and Garages) with the current design you’d have a hell of a time with wireless and analogue sensor inputs using a floating ground like this, Ive ran into that problem before trying to setup some aquarium automation that was near a Metal Halide lamp. The RF noise from the ballast resulted in erratic and worthless temp readings and flakey bluetooth communications; when I grounded the board properly those problems disappeared.
*edit* looking at the board you could attach the ground to the board mounting holes with some exposed copper and anyone needing to ground the board can attach to the mounting screw, simple and not really prone to user-hookup error.
PS: I noticed after the fact there’s a 5v header already on my OSbo; how much current of 5v would be safe to a small hub like this: http://www.monoprice.com/Product?c_id=103&cp_id=10307&cs_id=1030702&p_id=10065&seq=1&format=2 ? If you could get over 2A of power to a hub it’d open the doors to a whole bunch of USB options/modules.