Sorry Matt for the lingo – its hardware speak. 🙂 It was on at least two Mayfly 0.5b that I measured.!! I now measure the LiIon via the ADS1115 – here is what the two measurements look like in the field. A lot of work to get it there. They have the 4400mAh battery from Adafruit, and 3W solar panel.
Fancy that Turner Turbidity Plus sensor!. Interesting spec, says its supply is 3V-15V, but not clear what the wiper takes at 3V. It specifies the wiper at 12V@280mW (23mA?)
If you haven’t got an easy interface to the wires from the Mayfly, the “RS485 boards” could provide a reasonable interface to the wires (you wouldn’t need the separate RS485 hybrid)
If it needs the 12V for the wiper there is a range of Pololu boosters that fit on the board 12V. U3V12F12
If you want to switch directly from the LiIon bat, I have done this new board
Also I’ve put on an STC3100 Battery monitor that can measure V and mAHrs/Coloumb counter – looking interesting so far though, still a wip https://github.com/neilh10/STC3100arduino