lead acid car battery

Battery Monitor Circuit

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

Battery Monitor Circuit Diagram

battery monitor circuit diagram


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  • jayakant

    sir plz indicate the battery terminals

  • Roba

    I have simulated the circuit using multisim and it works well.Problem is that when normal voltage is supplied the LED indicating low battery still goes on with the normal voltage one,and when 16v(overvoltage)is applied all go on together with the overvoltage LED….how can i modify the circuit to switch off RED LED when normal voltage is applied,and switch off RED and GREEN when overvoltage is applied?

  • jerry

    this circuit is very usefull

  • ahsan

    thks sir mohan.
    for this ckt
    please tell me abt ur self.

  • dinesh

    dear sir,
    i visited the website today it was too good all projects are good with simple explanation,sir is it possible to make toy aircraft if so please reply me sir…..
    thank you.

  • Ganesh

    Hi Mr D. Mohankumar,

    Good day.

    Can I know the function and purpose for using capacitor C1 and resistor 10k ohm (r2&r5) and resistor 470 Ohm(R1&R4) in this circuit please. Thanks in advance.

  • czarz

    hi, Mohankumar, good day?
    same to you send by Neon, i need the circuit that will automatically cut off the charging process once the voltage is at the normal state…


  • ifeanyi

    pls i want a cut off circuit diagram attached to this circuit plssssssss

  • sarath

    Could you please tell whether the capacitor C1 in the above circuit mandatory?
    I just got a doubt as there was no capacitor across the battery in other battery charge monitoring circuits in this site.
    Please advice
    S Sarath

  • Neon

    Hello D. Mohan Kumar,
    I have built this circuit & it is working fine . But I want to extend this circuit . I want an- “automatic overcharging control with a Red LED indicator + this Battery charge monitor circuit” together .

    And in the Battery charge monitor circuit- Green LED is always on . That means it is taking some current and reducing Battery . In here I want Just a Red LED to indicate the low voltage 4.7V . The other Green & Yellow LED is not needed . And this circuit is always on . But the circuit will be trun on when the Load’s switch is on ( My load is 6V DC Motor for a Emergency fan )

    Sir, Please give me a above discribed circuit diagrm ( automatic overcharging control with a Red LED indicator + this Battery charge monitor circuit together ) and as far as less cost .

    I am using 6V Lead-acid Battery ( 6V 4.5Ah , Max voltage : 7.5V~ 7.9V, Stand-by use : 6.7V ~ 6.9V , Max Current : 1.35 A ) .

    – Thanks

    – Neon

    • Arsenic

      That’s because the monitor is measuring the tension delivered by the charger itself but not from the battery. It seems that the only way to make it work this circuit is with the charger disconnected.

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