Here is a project of a simple battery monitor circuit. The circuit will monitor the voltage of 12 and 9V batteries and indicate by activating an LED when the battery level will reach at the preset value. For example if you are using it for a 12V battery and you want the circuit to indicate when the
battery level will reach to 9V, 10V or 11V. Then you have to set this with the help of the 10K variable resistor used in the circuit.
This battery monitor circuit can be used for variety of batteries for example car batteries, other lead acid batteries, NiCd and NiMH batteries etc. The circuit is using only two transistors and few resistors. Both the transistors are working as switches, with a full charged battery the transistor Q1 will be in ON condition and Q2 will be in OFF. When the battery voltage will reach at the preset value set by the 10K variable resistor then the transistor Q1 will become switch OFF and Q2 will Switch ON and activate the LED.
By using a 3.3V / 1N746 or 4.3V / 1N749 zener diode the circuit can also be used to monitor 6V or 4.5V batteries.