Li-Ion battery status indicator LEDs with zener diodes
This is what you have .







simulate this circuit &ndash; Schematic created using CircuitLab

It is a perfectly good low bat indicator with a 5mm RED LED with 1k to 10k mcd @ 20mA so that at 1mA you get 5% of this.

In Zener datasheets, ESR is called Zzt at 20mA
In LED datsheets you estimate ESR from the VI curve @20mA and 1mA 
15 Ohms is typical for all 5mm LEDs but tolerance can be 50% leading to high Vf max. (worst case mfg tolerance) Some are lower tolerance from a good source.
All my custom sources are  10% but hard to find for you.