Also the MCU works down to 1.3v stability although it would not recommend it if reliability means anything.
I guess the key here is to _measure_ the consumption for an extended period of time. Do you have any means to do that ? I plan to do some tests with an ACS712, but not sure how to go that way.
Well, the battery drain time is highly variable (probably 20-50%),
and is not easy to detect precisely (if it goes dead at night...)
Some shunt system with analog integration and ... arduino recording should be better and a nice project, shouldn't it ?