So even though the whole micro-controller is still running as normal, the only practical purpose of the batteries is to retain the memory.
I think it keeps the RTC running too.
True - that's its main purpose.
There is no separate RTC circuit; the micro-control just has an internal 12MHz clock. I do not see an internal RTC circuit so I'm thinking it may be accomplished with code. Maybe under normal conditions the zero-crossing detector will regulate the time. From seeing the battery usage countdown timer working when unplugged it's safe to say the zero-crossing isn't the main controller of the time clock.
Therefore, macros with delays should still continue to run unless there is something in the code that halts macro & timer execution in the absence of a zero-crossing pulse. (TBD)
It would be a senseless waste of battery energy, but conserving battery energy may not have been very high on the designers' priority list.
Part of the PLC output circuit is powered by +30V that comes direct from the AC-DC bridge; so it looks like if it is trying to send PLC without AC, it's just sending TTL pulses from the micro-controller into a high impedance. The micro-controller typically draws 16mA.
The RF section is also connected to the same +5VDC line as the micro-controller. When not transmitting RF, the transmit section current will be negliable; and the receiver's IC only draws 4.5mA typically.
I wonder just how much, if anything, the USB"s own +5v power is used? I wonder if the micro runs off the USB +5v, regardless of batteries or AC? An easy test, just try plugging in the USB cable with no batteries and no AC, and see if the USB driver loads (make note to try this tonight).
The USB's +5V line is shown as Not Connected on the CM15A schematic.
Hmmm.... I hope the OP was referring to the the CM15A.
Nonetheless, lots of good questions and information being discovered on the topic.