Alphasense CO2, Power Issues

I found a schematic in your github repo, my suggestion would be to power everything external to the Mayfly with its own beefy power source, especially the relay bank.  It looks like you’re currently using the main 3.3v regulator of the Mayfly to power the relays and maybe some other things, which is probably what’s causing the brownout.  There’s a secondary 3.3v regulator that powers the SWITCHED_3V output (and the switched 5v and 12v circuits too), so that would be a better source for external devices (just leave pin D22 high all the time to leave the switched power outputs constantly on).  Most big external loads on the Mayfly should be connected to the switched outputs in order to provide a more stable voltage for the processor and other vital on-board circuitry.  Also, all of that power the Mayfly is providing to itself and all the peripherals has to run through the bq24074 charge controller, which handles all the power going into the Mayfly from either a battery or USB power (and also back out to the battery for charging), and sometimes it doesn’t like surges from motors and other heavy instantaneous loads.  So try powering that relay bank (and any other load) from an external source (making sure to tie all the grounds together for common reference) and see if that corrects your brownout issue.