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12V Lead Acid Battery Low Voltage Indicator

12V Lead Acid Battery Low Voltage Indicator

This is a simple circuit that will indicate a low voltage on a 12V lead acid battery. Many that have golf carts, small EV’s, RV’s, or solar power banks for homes have a number of 12V lead acid batteries to maintain and this circuit will help protect your investment while not using these batteries. When stored, batteries will self discharge, and if left for too long, will self-discharge to a damaging level. This circuit will light an LED when the battery voltage decreases to 11.6V. When the LED turns on, you’ know it is time to charge the battery to prevent the battery from dying on the shelf.

12V Lead Acid Battery Low Voltage Indicator Only a few parts are needed in the small but useful circuit. The BC557 PNP transistor controls the lighting of the LED. The BC557 transistors’ base is biased by the 10V zener diode. As long as the battery voltage stays above 11.6V, the zener keeps the base of the BC557 transistor high. When the battery becomes discharged, the zener stops conducting and the base bias goes low, and BC557 begins conducting, and the LED alerts you to the low battery voltage. You can use a variable resistor to fine tune the low voltage indication, and/or use a set resistor when you have a good value figured out.

Parts listing

RefDes Name Value Part Number Digi-Key Part Number Description
R1 RESISTOR 1K RMCF2010FT1K00 RMCF2010FT1K00CT-ND RES 1K OHM 3/4W 1% 2010
R2 VARIABLE 5K EVM-3YSX50B53 P3Y502CT-ND TRIMMER 5K OHM 0.15W SMD
C1 POLARIZED 10µF EEE-1EA100WR PCE3895CT-ND CAP ALUM 10µF 25V 20% SMD
D1 Zener 10V MMSZ5240BS-7-F MMSZ5240BS-FDICT-ND DIODE ZENER 10V 200MW SOD323
Q1 PNP 0 BC557BTA BC557BTACT-ND TRANSISTOR PNP 45V 100MA TO-92
L1 LED 0 VLMS1300-GS08 VLMS1300-GS08CT-ND LED SUPER RED 0603 SMD

12 Comments

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  • Ballemajn

    Why is the zener diode getting really hot while conducting (hot enough to melt solder tin) ? Connected to a 12V lead acid battery the circuit draw 0.127A when the LED is off, which I think is a lot, and only 0.017A when the LED is on.

  • Ebrahim

    Hi,it’s better to insert 1K resistor next to 5k pot to protect zener diode.

  • Edward Lousley

    Hi, I am wanting to make this for a 24v lithium Ion battery. can anyone suggest what adjustments need to be made to this circuit to make this work?

    Cheers.

  • Stick

    Akarsh, could it be because you are using a 5V source?

    This circuit is designed to work for a lead-acid 12V car battery.
    ~14V while charging, and this can illuminate the LED when it drops down past 11.6V.

  • Akarsh

    i tried this circuit on multi sim. Didn’t work. The led glowed when voltage was high and didn’t glow when voltage was low, instead of the other way round. Any ideas why?

  • 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.

  • Name *

    Were i buy voltage indicator 12v

  • Artis

    How about 3 stage or 4 or any other number voltage indicator using transistors?
    I lost my circuit it was Russian one old, can some one draws one? I can adjust it myself. ty.

  • hassan

    niceeee site wish to get all new from.you

  • Ulf Larsson

    Simple and nice circuit but what happens when you have 12V or more on battery and adjust the variable resistor to zero ?
    You get smoke from pot or zenerdiode because you force 12V on 10V zenerdiode.

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