I recall quite some time ago working out some low voltage reference, and obtaining all manner of voltages < 5.1V zeners. 1watt zeners needed at least 12mA for
to operate at their fixed zener voltage, 0.5watt zeners are a little better at 4-6mA....
Your 680R is too high, use 470R
You can improve it by replacing the resistor with a constant current source.....
Anyway plain zener diodes for low voltage reference are a waste of time:wink:, and poor temperature co-efficient.
Use Voltage reference devices such as LM335-2.5V, this will give you an accurate and stable reference and using < 0.5mA.
There's others that use uAmps....
Being battery operated, you do want to consume least amount of current, don't you ?