BATTERY MONITOR CIRCUIT FOR 12 V LEAD ACID BATTERY

            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 components required are

• BC547 Transistor
  
   
• Zener Diodes(Values re specified in circuit diagram)
  
• LEDs(Red,Green and Yellow)
  
• Resistors(Values are specified in the circuit)
• Capacitor(Value is specified in circuit)
• 12V lead-acid battery
  
• Breadboard or PCB

CIRCUIT DIAGRAM

          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 Z1 and Z2 cease to conduct and Red LED only lights indicating low battery condition. If the voltage is between 12 volt and 14 volts, Zener diode Z1 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 Z2 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