Here is a simple Battery Monitor circuit for a quick check of 12 volt Lead-Acid Battery. Battery charge should be constantly monitored to increase the life of the battery. Overcharge as well as under charge will reduce the battery life. The terminal voltage of the Lead Acid battery should be with in the range of 12.5 to 13.5 volts. If the battery voltage reduces below 10 volts for long period, battery will not accept any charging current. Similarly if the terminal voltage exceeds above 14 volts, battery will be destroyed.
The circuit is a Zener controlled transistor switch lighting three LEDs Red, Green and Yellow to show battery states like Low, Normal and High. When the battery voltage is less than 11 volts, Zener diodes ZD1 and ZD2 cease to conduct and Red LED only lights indicating low battery condition. If the voltage is between 12 volt and 14 volts, Zener diode ZD1 forward bias and T1 conducts. The Green LED connected to the collector of T1 lights indicating normal voltage. If the battery voltage exceeds 15 volts, Zener diode ZD2 also conducts and T2 forward bias. This lights Yellow LED indicating over charge. Thus the following indications can be obtained
Red Low voltage
Green Normal voltage
Green and Yellow Over voltage