These are Five Battery low voltage alarm indicator circuits, as ideas to make small projects, them use LED display easy, cheap with normal parts look below [...]
The simple low voltage tester circuit using LM339,can use Battery voltage monitor for voltage sources that problems, by LED display and alarm sound.
Chayapol (my son) tried to use battery tester, very convenient. He can make it easy to use 1.5V battery of all size(AAA,AA,D and more) and 9 volts battery.
This fine 1.5V battery tester circuit using LM324, Can measure the voltage of the 1.5V battery AA or AAA. As decimal point. Easy to use by LEDs display
Simple battery status monitor circuit using IC-7555 or low power LED flasher by ICM7555, need to use LED display status of the battery 6V-12V
If you want to teach kids to learn simple LED circuit. This is one of good circuit. It is the Simplest On-Off battery indicator using two LEDs.