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36V Battery Level Indicator circuit

36V Battery Level Indicator

This battery level indicator offers (5) LEDs that light up progressively as the battery voltage increases. This is a update of the 24V Battery Level Indicator.
While designed for 36V systems, it is easily modified to 24V, 48V or 60V simply by changing two resistors.

LED Color Charge Level
Red: Power Connected (0%) (essentially always on)
Orange: Greater than 35V (25%)
Yellow: Greater than 37V (50%)
Green: Greater than 39V (75%)
Blue: Greater than 41V (100%) (full charge is about 41 to 42V)

Of course, you may select your own colors if desired.

Schematic of the 36V Battery Level Indicator Circuit

36V Battery Level Indicator circuit

High voltage issues

One limiting factor is the LM339 that has an absolute maximum voltage rating of 36V –and it is not good practice to operate near that point. The solution involves running the IC power rail off a zener shunt regulator. Shunt regulators are very simple, inexpensive and robust –good for this application. However, the LM339 open collector outputs cannot drive LEDs powered from the battery bus due to the same maximum voltage limitation. There are two solutions for this: increase the current rating of the shunt regulator so that it can power the LEDs, or run the LEDs via a cascode amplifier arrangement –I chose the cascode amplifier.

Cascode amplifier

A cascode amplifier is a configuration where one transistor feeds a 2nd transistor that is connected in the common base configuration. This configuration is generally used for RF amplifiers. In this circuit it offers one great property –low voltage in, high voltage out. By tying the bases of these transistors to the 13V bus, the LM339 open collector drive transistors never see more than 13V. However, the cascode transistors may drive LEDs that are tied to a much higher voltage –up to 80V using the MPS-A16. By limiting the emitter current, the LED current is automatically limited to the same value. While this is relatively busy, it places virtually no burden on the shunt regulator.

Circuit Operation

D1 is the voltage reference zener. Tied to this is a string of divider resistors (R2-6) that set the various fixed voltage levels. R7 & 8 form a voltage divider to that divides the battery voltage by a factor of 9. The quad comparator compares the various voltages from the two dividers.

For calibration, connect to a voltage source that can be set to the highest LED threshold (41V in this case). Then adjust the calibration pot until D2 flickers. The remaining LEDs will switch on close to the indicated voltage –accuracy of those voltages may suffer slightly, but should be close.

The LEDs are biased to operate at 2.3mA which is reasonably bright for high efficiency LEDs. This current can be adjusted simply by varying the emitter driver resistors (R9 through R13). To reduce standby power, a push-to-test pushbutton may be used.

Bug

Testing on variable voltage turned up an interesting bug –when the voltage was turned down to about 10V, all LEDs lit again. This was traced to Zener D9 dropping out of conduction thus starving the voltage divider. Connecting a 33K resistor across D9 corrected this little problem without affecting anything else.

Photos

17 Comments

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  • morgan

    Hi. Can I add a 12 volt relay for alarm on this 24 v batt level indicatator
    Circuit.any suggestion n how to add relay.
    Thnx

  • why?

    hey,
    this circuit seems like perfect for my project,
    all i want to know is that will this circuit take up 30 amps of current and more?
    as i am gonna use this in my ekart which is powered by 12v,28Ah battteries connected in series to get 36v , 28Ah ,
    what do you suggst?

  • Carlos

    Thanks for sharing this circuit, Jim.

    Some calculations required, so I could replace a couple of “less common” value resistors, using the ones I had in my spares.

    Built in stripboard and working flawlessly, as a “fuel” gauge in my kids 36V electric scooter.

    Thanks!!

  • Moynul

    Thanks for your reply.

    What do you mean by –
    >>Click on the schematic to open it, then copy and paste it to Word, Paintbrush or other drawing program–then it may be saved and/or printed.

    I wanted a 72V Battery Level Indicator circuit diagram. You already told that “36V Battery Level Indicator” not applicable for 72V. So, please send me a working 72V Battery Level Indicator.

  • Moynul

    Thanks for your reply.

    Please send me a 72V Battery Level Indicator.

    • Jim Keith

      Click on the schematic to open it, then copy and paste it to Word, Paintbrush or other drawing program–then it may be saved and/or printed.

  • Jim Keith

    woops or whoops is an undocumented idiom
    It indicates a mistake.
    In your case (72V), this does not apply.

  • Moynul

    Thanks but sorry what do you mean by “woops, it should indicate ÷6 for 24V”

  • Moynul

    Thanks Keith for this circuit. What change should I make for 72V?

    Please send me the circuit diagram for 72V to my email.

  • Edward Lousley

    C1 is given a value on 0.1, is this 0.1 Micro Farads?

    • Jim Keith

      In my schematics, if not otherwise indicated, capacitor values are in uF (micro-farads). Another relatively common standard is nF (nano-farads). With a little experience, one can easily guess the unit of measure.

  • Chuck

    HA! nevermind I was looking for a 13.15k resistor. and I misread it. its R13 a 15k resisitor. I feel silly now. I’ll post some pics of the build.

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