Confused about battery voltage warning circuit with PNP transistor and Zener diode
I've simulated the circuit you've posted and it doesn't seem to work at all.

The problem, I'm sure, is at least partly due to the emitter resistance killing the gain of the circuit. 

I've redesigned it and made the first stage NPN common emitter with a pot to adjust the low voltage switching point, and added the second stage to invert the output of the first stage, isolate the LED load, and snap the LED ON quickly at the switching point.

I've shown a white LED because that's what I ran the simulation with, but any garden-variety LED will work by adjusting its forward current via R3.

In the graphic, below, the red trace is the battery voltage decaying from 12 volts to zero volts, and the green trace shows the LED popping ON at about 8.9 volts and then getting dimmer and dimmer as the battery voltage decays.