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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.
https://monitormywatershed.org/sites/TUCA-Na13/
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)
https://github.com/EnviroDIY/SensorModbusMaster/tree/master/hardware/Modbus-Mayfly_WingShield
If it needs the 12V for the wiper there is a range of Pololu boosters that fit on the board 12V. U3V12F12Â
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
