For the past two days, I've had an Adafruit Feather Lora (915MHz) and battery running, broadcasting out it's battery voltage. This is measured through a 100k/100k voltage divider fed into an analog pin. (The on-board circuit already has this included on D9.) I had to attach two wires so that DIO0/DIO1/DIO2 would all be hooked up. (I'm still not sure that all three are required, but all three are at least specified in the arduino-lmic library I'm using.)
I wanted to see how long this module could last on battery power and measure the voltage at the same time. I've had this running for around two days thus far and it's still going. (My initial computation showed it should last just under 4 days.) I haven't done any power optimization yet. Once I do, this should allow it to last MUCH longer. The SAMD21 on the board has an RTC which allows you to put the module to sleep. Right now, it's actually been staying awake during these 2 days just broadcasting a LoRa message to The Things Network every 2 minutes to comply with the fair usage policy. My total radio time is around 46 milliseconds each message. (SF7) This is from broadcasting just two bytes of information. The first byte is an 8-bit ADC reading from the battery voltage and the second byte is a pseudo-random value.
0x92 = (146 / 255 * 6.6) = 3.7 volts