You do in fact have the “DS” (Discrete Sample) sketch - the last line of the screenshot was covered up by the caption - and that’s where the difference is. The test output is the same for both.
In that case, the battery voltage is reported in the “Vrms” slot, which is the 5th variable, 3.48 in your case. It switches to reporting battery voltage if no a.c. input is detected at start-up. 3.48 V is OK for rechargeable cells, but low for alkaline.
The optical sensor takes no current normally, but (according to the shop web page)
Current Consumption - Pulse: 7.5mA @5V for duration of pulse (standard meter 100ms)
3 new alkaline cells should be able to support that, so I’m rather struggling to see why it’s falling over. Unfortunately, my emonTx V3.4 and optical pulse sensor are in use for a test for the next few weeks, so I can’t make any measurements using it.
Are you able to give it a temporary mains supply? The reason I’m keen to see what happens then is there are two separate power supplies: the 5 V USB, FTDI programmer and battery holder feed into a MCP1702 regulator connected directly to the 3.3 V used by the processor and radio, whilst the a.c. adapter feeds into a MCP1754 that feeds into the 3.3 V via JP2, the link right next to the battery holder.
I’m starting to wonder whether you have a high resistance joint somewhere - either in the battery holder itself or a badly soldered joint on the board.