I have a 6 X 2V (400Ah) batteries in the office and I use usual DC to AC inverter to run our computers. One problem that we have is there is no way to know how soon our batteries are going to die. I got usual volt meter, but I need to monitor that all the time. What can I do to make a beep when a battery volts goes below 11.5? I think it is very simple thing but I don't know what it is called or how to make it.
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.
A quick search returned a few circuits that may be better. I had originally built my circuit based on the second link.
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).