That is useful, and along the lines of my thinking: moving to LiSOCL2. Though I am having trouble getting supplies to test. The internal impedance of those, especially when low or cold, is a big question to resolve. My system can use all the deep-sleep tricks, but when measuring the load averages ~50 mA with spikes up to ~350 mA for fractions of a second, and I wonder if this will cause a brown-out with the LiSOCL2 cells. Additionally, if I use these cells, we’ll need to use an array of 4, behind protection diodes, which drop the input voltage a bit, but also could provide more current during operation.
I had not thought to try using a bigger buffer capacitor in front of the Mayfly. I got some more stability with 1000uF with my ammeter in the circuit, but some configurations still got resets: I figured that it was confusing the voltage regulator somehow, as I could not resolve a deep voltage sag on the inputs when measuring. I will look at the datasheet again and meditate.
But if I could just set the brownout to 1.8V instead of 2.7V that would still be a nice test/check. I tried hooking up through the SD card vertical socket and the DTR pin of the FTDI connector, but wasn’t successful. I am wondering if the pin order in J7 is correct in the schematic, I can’t see the traces under the SD card slot on the PCB, so hoping that I can get clarity here, though I will probably just try to probe out the pins soon.