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Battery Self Discharge Indicator

If the lead acid battery is not using for long time, self discharge takes place at the rate of 4 % per week at 27 degree. For example, a 125 Ah tubular battery self discharge at the rate of 5 Amps current per week if it is not in charge / discharge cycles. This add on circuit can be used to detect the self discharge of the 12 volt lead acid battery below the safe level of 11.6 volts. It is ideal to monitor the charge level in the battery, it the battery is not using for long period.

The Battery Self discharge Indicator circuit uses only a few components and its working is simple. A PNP transistor T1 act as a switch to light the LED, if the battery voltage drops below the safe level. The base bias of T1 is controlled by a Zener diode ZD. Its rating is 10 volt 1 W. The Zener diodes usually requires 1.6 volts excess than its rated value to enter into the “Avalanche state”.

Battery Self discharge Indicator Circuit

Battery Self discharge Indicator Circuit

Experiment of discharging a 6V 4.5Ah lead-acid battery


Relay contact of solid state switch

So, as long as the battery voltage is above 11.6 (10+1.6), Zener conducts and keeps the base of T1 high. Since T1 is a PNP transistor, it will not conduct till its base becomes negative. So LED remains dark. When the voltage in the battery reduces below 11 volts, Zener turns off and the base of T1 becomes negative. T1 then conducts and LED lights. So the battery can be charged again to keep it in top condition. Preset VR can be used to set the exact point at which LED turns on.

9 Comments

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  • Sebastian

    This voltage indicator, shall work well with high input impedance transistors (JFET, MOSFET). Otherwise, the BJT will source some current to the trimmer and the voltage drop across the zener will not serve as a reference for activation or deactivation. The circuit is beautifully simple and functional. I would just add this correction.

  • Kisha

    Stunning quest there. What happened after? Take care!

  • Donald

    Hi there to every one, it’s in fact a good for me to
    go to see this web page, it consists of useful Information.

  • akash

    Hello sir
    This type all circuit led is not on immediately
    Because transister on .1-.7v when battery voltage is low off full charge led glow with low power

  • Yongs

    In my case, it is not working (circuit: Battery Self discharge Indicator Circuit
    ). What is a checking point?
    If I increase the voltage to the 11V7, the LED was on.
    If I decrease the voltage to the 9V, the LED was off.
    I have plugged the ZD with IN4740A number.
    I have Plug the T1-BC557 with CBE pin layout.
    Sure, it was tested on the breadboard.

    What is wrong?

  • satishkumar

    Hi Mohan,
    I am currently designing a circuit which shall discharge the Coin cell battery(CR2032) to a Low battery threshold voltage value(configurable) to test my target system.Will i be able to scale this circuit for my above said application.Plz advice me.

  • jayasurya

    thanq its nice

  • D.Mohankumar

    Thanks for the feed back.Use a pen drive or portable hard disc for storing files. It will be useful to keep the info as a library

  • Ed

    Thank you for all the fine circuits, you have sent and kept us all up to date,,I have been downloading them to my files,,though my burner isn’t working,,don’t know and am tired of spending,,”WASTING” $ having these things fixed or replacing replaceing,,It has crashed afew times and I lost much Personally VALUABLE circuit info and componet data, but thankfully it is still out there and am able to re locate it If I can remember the sites or the project, so thats good as long as I have the internet and access,, sure do miss Libraries like they use to have, when they stocked the shelves with this info and there was no internet or computors to have to rely on.. BUT a big THANK YOU D.Mohankumar

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