I was planning on setting a threshold VCC limit to prevent transmission from sinking my coin cell below safe voltage level. I.e, check VCC before TX, cancel TX if the test fails.
Problem is, voltage drops due to TX likely aren't deterministic, so you need to give yourself a conservative buffer. Made me wonder if VCC could be monitored DURING transmission, and transmission can be aborted in case of trouble?
From RFM69.cpp:
setMode(RF69_MODE_TX);
uint32_t txStart = millis();
while (digitalRead(_interruptPin) == 0 && millis() - txStart < RF69_TX_LIMIT_MS); // wait for DIO0 to turn HIGH signalling transmission finish
setMode(RF69_MODE_STANDBY);
^--- within that while loop seems like the logical place to be checking. I don't know how long TX takes versus VCC measurement, so perhaps there's no time for it? This is several layers below anything I've worked on with microcontrollers/radios however, so please use kid gloves when explaining why it won't work