@neilh20 so the board here is called a MKS Base V1.4 if i remember correctly. It’s an old 3d printer board. There are a slough of options and which is used really doesn’t matter. They are all, like the mayfly, a modified arduino. Though these use the arduino mega as their main design.
I have the relay controlling the power to the board itself. The power source is still TBD. These board generally take in 12v or 24v. A separate charge controller and 12v battery could make quick work of this task. Though if there are regular maintenance visits, then there is potential to just swap out batteries. The battery basically just needs to sit idle, until sampling, in which case someone should be on the way to pick it up. battery swaps can occur then too. Just off the top of my head I’m thinking maybe a 12v 5ah battery, or the very easy to get 12v 7ah battery. Another option might be tool batteries with and adaptor. This would make swapping things out very quick and wont allow for it to be hooked up incorrectly. Something like this.
This is a lot of spit balling.