3.0v is the standard battery voltage. The consumption with the calculator on and no program/no key pressed is below 1 mA - so it shows as zero on the screen.

The image shows the consumption when a key is pressed: 15 mA (when voltage is 3.01 - we will see that current depends on voltage too)

This is the minimum voltage that allows th calculator to work with keys pressed. Notice that the consumption is higher: 18mA when a key is pressed, still "under the radar" for calculator merely on.

There are lower voltages that still show the screen contents, but that will not support a key pressed, and will go blank. Reset does not work to bring it back to life - you need to cut power and power back again (at a safe voltage > 2.6v

There are lower voltages that still show the screen contents, but that will not support a key pressed.

I managed to lit the low battery indicator while playing with the voltages, but not always! it is quite difficult to get, and it may have happened at lower voltages.

As explained, when no key is pressed, the consumption is "below the radar" - hence the long duration of the batteries.

The internal resistance of the CR2032 batteries is around 20 ohm - so for two batteries in parallel the resistence will be 10 ohm and the drop of voltage with 18 mA is 0.18v - which may be able to wreak havoc when the battery levels are low already.

Also this is the graph for Varta batteries; nondescript, no-brand batteries may have much higher internal resistence and hence higher voltage drop.