Create a beeper when a battery voltage goes down to specific number
Here is a circuit I made some time back and it was very useful. The Zener value can be roughly between 7v and 10v. The 555 IC acts as a comparator and turns on the output at pin 3 (OUTPUT) when pin 2(TRIGGER) is less than 1/3 Vcc. Adjust the Potentiometer(R1) so that the output is turned on at 11.5 volt.







simulate this circuit &ndash; Schematic created using CircuitLab

A quick search returned a few circuits that may be better. I had originally built my circuit based on the second link.

http://www.aaroncake.net/circuits/lowvolt.asp

http://www.homemade-circuits.com/2013/05/simple-low-battery-indicator-circuit.html

For the second one, you can replace the LED with a buzzer. 

For all circuits, select a buzzer that works when directly connected to a voltage (ie: make sure it is not a simple piezo disk or speaker).