Low Voltage indication for ProMini -3.3V/8Mhz

I have a PCB designed for the ProMini 3.3V version ... this has only the Digital Pins broken out on connectors.

The PCB itself is powered by a single LiPo cell after being boosted to 5V as I have to use the raw Vin input.

Now I would like to inform the user when the battery needs recharging.

I am using a MT3608 module from E-Bay. It is actually supposed to use a B628 regulator but I see its using a B6287L . I have no clue what the 7L means.

This module is supposed to regulate from 2V onwards... by which time the LiPo is dead and gone.

In such set ups is it possible at all to find battery status without actually measuring it by a Analog input pin ?

Why are you wasting power by boosting the cell voltage?
The ATMega328 will run on as little as 1.8 V (at lower clock speed).

jremington:
Why are you wasting power by boosting the cell voltage?
The ATMega328 will run on as little as 1.8 V (at lower clock speed).

Hmm... that s a good line of thought. There is a MFRC 522 RFID reader connected but that also I think will run at 2.5V . But right now the hardware is rigged for 5V input and to modify will need some surgery.

But right now the hardware is rigged for 5V input

What hardware?