Automatic 12V Battery Low Voltage Cut-Off Circuit Using LM358

Protect Your Battery | Prevent Deep Discharge | Simple & Reliable

Introduction

A low voltage cut-off circuit is an essential protection system for 12V batteries. Deep discharging a battery can significantly reduce its lifespan, performance, and charging capacity. This is especially important in battery-powered systems such as solar power systems, UPS units, inverters, portable power supplies, and automotive applications.

In this project, we will discuss a simple and reliable Automatic 12V Battery Low Voltage Cut-Off Circuit using LM358. The circuit continuously monitors the battery voltage and automatically disconnects the load when the battery voltage falls below a preset level.

When the battery voltage rises again after charging, the circuit can reconnect the load automatically, depending on the circuit configuration.

This project is easy to build, inexpensive, and suitable for electronics hobbyists and DIY enthusiasts.

Circuit Diagram

The circuit uses an LM358 operational amplifier as a voltage comparator.

The battery voltage is monitored using a resistor divider network. A reference voltage is provided to one input of the LM358, while the battery voltage is connected to the other input.

When the battery voltage is above the preset level, the comparator output activates the transistor and relay, allowing power to reach the load.

When the battery voltage drops below the preset level, the relay switches off and disconnects the load.

Main Circuit Components

• 1 × LM358 Operational Amplifier
• 1 × BC547 or 2N2222 Transistor
• 1 × 12V Relay
• 1 × 1N4007 Protection Diode
• 1 × 10K Potentiometer
• Resistors
• LEDs for Status Indication
• PCB or Veroboard
• 12V Battery
• Load

---

Components List

Sr. No.	Component	Value / Specification	Quantity
1	Operational Amplifier	LM358	1
2	Resistor R1	100KΩ	1
3	Resistor R2	10KΩ	1
4	Resistor R3	10KΩ	1
5	Resistor R4	47KΩ	1
6	Potentiometer VR1	10KΩ	1
7	Transistor Q1	BC547 / 2N2222	1
8	Relay	12V SPDT	1
9	Diode D1	1N4007	1
10	LED	Red / Green	2
11	PCB / Veroboard	General Purpose	1

---

How the Circuit Works

The LM358 is configured as a voltage comparator in this circuit.

A portion of the battery voltage is applied to one input of the LM358 through a resistor divider network. The reference voltage is set using the potentiometer.

The LM358 compares the battery voltage with the preset reference voltage.

When Battery Voltage Is Normal

When the battery voltage is above the preset cut-off level:

• The LM358 output changes state.
• The transistor receives the required drive signal.
• The relay becomes energized.
• The load remains connected to the battery.
• The green LED can indicate normal battery operation.

When Battery Voltage Drops

When the battery voltage falls below the preset level:

• The LM358 comparator changes its output.
• The transistor switches off.
• The relay becomes de-energized.
• The load is disconnected automatically.
• The red LED can indicate low battery voltage.

This prevents excessive battery discharge and helps protect the battery from damage.

Setting the Low Voltage Cut-Off Level

The cut-off voltage can be adjusted using the 10K potentiometer (VR1).

For a standard 12V lead-acid battery, a typical low voltage cut-off level can be set around:

11.0V to 11.5V

The exact voltage depends on:

• Battery type
• Battery capacity
• Load current
• Application
• Recommended discharge level

For example, a solar battery system may require a different cut-off voltage compared to a portable battery-powered device.

Always check the battery manufacturer's recommended discharge voltage before setting the circuit.

Applications

This automatic battery protection circuit can be used in many electronics projects.

1. Solar Power Systems

The circuit can disconnect loads when the solar battery voltage becomes too low.

2. UPS Systems

It can protect backup batteries from excessive discharge.

3. Inverters

The circuit can be used as a low battery protection system for small inverter projects.

4. Car and Vehicle Batteries

It can monitor battery voltage and disconnect auxiliary loads.

5. Portable Power Supplies

Useful for DIY battery-powered projects and portable electronic devices.

6. Emergency Lighting Systems

The circuit can help protect batteries used in emergency lighting applications.

Advantages

This circuit offers several advantages:

• Simple and easy to build
• Low-cost components
• Protects batteries from deep discharge
• Adjustable cut-off voltage
• Uses a commonly available LM358 IC
• Suitable for DIY electronics projects
• Can be used with different battery-powered systems
• Provides automatic load disconnection
• Easy to modify and upgrade

Important Safety Precautions

When building this circuit, keep the following precautions in mind.

Use a Proper Relay

Select a relay according to the voltage and current requirements of your load.

Check Component Ratings

Make sure all components are rated correctly for the battery voltage and load current.

Use a Flyback Diode

Always connect a protection diode across the relay coil to protect the transistor from back EMF.

Check Battery Polarity

Incorrect battery polarity can damage the LM358 and other components.

Test Before Connecting the Load

First test the circuit using a multimeter before connecting expensive equipment.

Use a Suitable Power Transistor

For higher current applications, use a suitable transistor or MOSFET instead of a small-signal transistor.

Possible Improvements

This basic circuit can be improved in several ways.

Add Automatic Reconnection

The load can automatically reconnect when the battery voltage rises above a preset recovery voltage.

Add Hysteresis

Adding hysteresis prevents the relay from rapidly switching on and off near the cut-off voltage.

Add a Digital Display

A voltage display module can be added to monitor the battery voltage in real time.

Replace the Relay with a MOSFET

For silent operation and improved efficiency, a MOSFET can replace the mechanical relay in suitable applications.

Add Overcurrent Protection

A fuse or electronic protection circuit can be added for additional safety.

Troubleshooting Tips

Relay Is Not Switching

Check:

• Battery voltage
• LM358 power supply
• Transistor connections
• Relay coil voltage
• Diode polarity

Cut-Off Voltage Is Incorrect

Adjust the potentiometer slowly while monitoring the battery voltage using a multimeter.

Relay Keeps Switching

This may happen when the battery voltage is close to the cut-off point. Adding hysteresis can solve this problem.

LED Is Not Working

Check:

• LED polarity
• Series resistor
• Output voltage
• Wiring connections

Frequently Asked Questions

Can I use another op-amp instead of LM358?

Yes. Other suitable comparator or operational amplifier ICs can be used depending on the circuit design. However, the LM358 is popular because it is inexpensive and widely available.

Can I use this circuit with a 24V battery?

Yes, but the resistor divider network and voltage reference must be modified according to the 24V system.

What is the recommended cut-off voltage for a 12V battery?

It depends on the battery type. A typical protection level may be around 11V to 11.5V, but you should always follow the battery manufacturer's recommendations.

Can I use a MOSFET instead of a relay?

Yes. A MOSFET can provide faster and silent switching, especially for DC loads.

Can this circuit protect a lithium battery?

The basic circuit can monitor voltage, but lithium batteries require accurate cell protection and balancing. A proper BMS is recommended for lithium battery packs.

Conclusion

The Automatic 12V Battery Low Voltage Cut-Off Circuit using LM358 is a simple and useful project for protecting batteries from deep discharge.

The circuit continuously monitors the battery voltage and automatically disconnects the load when the voltage falls below a preset level. This helps improve battery life and protects battery-powered equipment.

Because the circuit uses inexpensive and commonly available components, it is an excellent project for students, electronics hobbyists, and DIY enthusiasts.

With additional modifications such as hysteresis, automatic reconnection, MOSFET switching, and digital voltage monitoring, this circuit can be adapted for many practical battery protection applications.

Tweet