3 LED Battery Voltage Level Indicator Circuit

A healthy battery is important for the car or any machine with a battery to run well and avoid problems.

This post shows how to build a tool that checks the battery voltage and informs if it is low, good or overcharged.

It works with most 12V car batteries but with a small change one can even use it on 24V systems.

The tool uses a special chip called a IC LM393 to measure the voltage accurately and light up LEDs to show you the batteries health.

What is a 3 LED Battery Voltage Level Indicator Circuit:

A simple electronic circuit known as a 3 LED battery voltage level indicator circuit indicates the voltage level of a connected battery visually.

Many battery powered devices use this kind of circuit to let consumers know how much charge is left in their battery.

In general different voltages or battery conditions are shown by this LED indicators circuit.

How to Build:

Parts List:

Circuit Construction process is mentioned below:

• Assemble the necessary components and be sure they are functional.
• Verify again that the LM393 potentiometers, LEDs, resistors and Zener diode specifications match the design.
• On the PCB place the LM393 dual operational amplifier.
• The PCB should be connected to the positive and negative power supply connections.
• Connect LM393s pins 4 and 11 to the positive and negative power supplies respectively.
• Connect LM393s pins 2 and 4 to potentiometer P1.
• Connect LM393s pins 6 and 4 to potentiometer P2.
• As shown in the circuit diagram connect the resistors and LEDs.
• Between pins 2 and 7connect a yellow LED.
• Between pins 6 and 7 connect a green LED.
• Between pins 2 and 12 connect a red LED.
• Connect the Zener diode between LM393 pins 8 and 4.
• If using a standard operational amplifier like LM358, LM1458 or MC4558 connect the series resistances for LEDs 2 and 3.
• The PCBs positive and negative terminals should be connected to the controlled power supply.
• Activate the power source and adjust the voltage range as per the preference.
• At the low battery threshold which is between 9.5 and 10V adjust potentiometer P1 until the yellow LED illuminates.
• To ensure that the battery is properly charged adjust potentiometer P2 until the green LED illuminates.
• Increase the voltage even further to make it look like overcharging then adjust P2 till the red LED illuminates at about 14 to 15V.
• Use a suitable fuse to connect the device in order to avoid possible dangers.

Testing:

• Connect the indicator to the real battery system to confirm its operation.
• Be sure the LEDs react correctly to the batteries condition.

Warning:

• To prevent damage to the indicator take extra care during the building and installation stages.

Formulas:

Designing a 3 LED battery level indicator circuit with LM393 IC involves the following calculations for resistors:

Voltage Divider Resistors:

These resistors create the reference voltages for the LM393 comparators which decide when each LED switches ON.

To calculate them you will need following formulas:

• Battery Voltage (Vbat): Minimum and maximum voltage of the battery.
• Number of LEDs (N): here N = 3.
• Desired Threshold Voltages (Vt1, Vt2): Voltages at which each LED should turn ON (slightly higher than LED forward voltage).

Current Limiting Resistors Rled:

To avoid damage these resistors limit the amount of current that passes through each LED.

What one will need is below mentioned formulas:

LED Forward Voltage (Vf): Found in the LED datasheet at a specific current e.g. 20mA.

Desired LED Current (Iled): Which is between 10mA and 20mA.

The formula uses ohms law:

R_led = (V_bat – V_f) / I_led

where,

• Vbat is about 3.7V
• VfLED1 is 2.0V example LED forward voltage
• Iled is 15mA required current for LED
• Rled is 3.7V – 2.0V / 15mA
• Rled is 1.13k one should choose a close standard value like 1.1k

Remember:

These are shorter examples for information on specific pin connections and biasing specifications see the LM393 IC datasheet.

Calculations for resistors and voltage dividers are made simpler with online calculators.

Circuit Working:

The IC LM393 dual operational amplifier is the circuits essential component.

Potentiometers P1 and P2 are set up to measure the input voltage.

Potentiometer P1 sets the voltage threshold which is between 9.5 and 10V at which the second LED turns green indicating a fully charged battery.

The third LED on the other hand indicates overcharging at 14 to 15V when P2 sets the threshold for it to light up.

The Zener diode supplies the operational amplifiers reference voltage ensuring exact voltage readings.

Yellow LED Low Battery: Only the yellow LED illuminates when the battery is low.

Green LED Good Battery: Both the green and yellow LEDs illuminate when the battery is fully charged.

Red LED Overcharging: If the battery experiences overcharging all three LEDs light up.

LED Connection:

• LEDs are connected in series to maintain a constant current ensuring that the current does not increase when multiple LEDs lit at the same time.
• If a classic op amp is used e.g. LM358, LM1458, MC4558 series resistances are added to LED2 and LED3 to maintain proper functionality.

Calibration:

• The calibration process involves adjusting potentiometers P1 and P2 with a controllable power supply.
• P1 is adjusted until the yellow LED lights up at the low battery threshold.
• P2 is adjusted until the green LED lights up at the required voltage indicating a well charged battery.
• Further increasing the voltage allows P2 adjustment until the red LED lights up indicating overcharging

Warning :

• The device must be connected to a suitable point in the battery circuit and only through a suitable fuse to prevent fire or damage to the circuit.

Final Testing:

• Once calibrated the indicator is connected to the actual battery supply for final testing.
• LEDs should respond accurately to the battery conditions providing a visual indication of the battery status.

Conclusion:

Finding the batteries overall charging status should be easy and quick and which is possible by using a 3 LED battery voltage level indicator circuit.

Depending on the battery voltage LEDs with different voltage thresholds can provide a low, medium or strong visual signal.

References:

Battery Level Indicator 

Battery indicator