🔋 Build an Auto Cut-Off 12V Battery Trickle Charger

Looking for a smart way to charge your 12V battery without overcharging? This project shows you how to build a DIY 12V trickle charger with an auto cut-off feature using the widely available NE555 timer IC and a few basic components.

Perfect for lead-acid batteries, this circuit will automatically stop charging when the battery reaches full voltage, extending battery life and reducing energy waste.

🔧 Components List (Line by Line)

NE555 Timer IC – 1 pc

• NE555 Timer IC – 1 pc

1N507 Diode – 1 pc

• 1N507 Diode – 1 pc

1NB62 Zener Diode (6.2V) – 1 pc

• 1NB62 Zener Diode (6.2V) – 1 pc

D36 Diode – 1 pc

• D36 Diode – 1 pc
• 10µF Electrolytic Capacitor – 1 pc
• Resistor 1kΩ – 1 pc
• Resistor 62Ω – 1 pc
• Resistor 4.7kΩ – 1 pc
• Resistor 10kΩ – 1 pc
• Resistor 12kΩ – 1 pc
• BC547 NPN Transistor – 1 pc
• 12V SPDT Relay (e.g., 650-1) – 1 pc
• Dual Color LED Indicator – 1 pc
• Freewheeling Diode – 1 pc
• 15V DC Adapter or Solar Panel – 1 pc
• Optional: Terminal blocks, enclosure

🧠 Circuit Overview

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This project uses the NE555 IC as a voltage-level detector. When the battery voltage is below a set threshold, the output from the NE555 is high, which activates a transistor-driven relay to start charging the battery.

Once the battery is fully charged (typically around 13.7V to 14.2V), the NE555 detects the voltage via a resistor divider, switches the output low, and disconnects the charger via the relay, cutting off power to the battery.

⚙️ How It Works

🔌 Power Input & Regulation

• The 15V DC input is passed through protection diodes and a Zener to stabilize the voltage.
• This prevents reverse polarity and voltage spikes.

🧪 NE555 Comparator Mode

• Configured in comparator mode, the NE555 reads the divided battery voltage.
• If the voltage is below the threshold, the output goes HIGH, turning ON a transistor.

🔁 Relay Control

• The transistor switches the relay, which connects the charging source to the battery.
• A flyback diode is used to protect the transistor from voltage spikes.

🔋 Auto Cut-Off Logic

• Once the battery voltage exceeds the set point, the NE555 output goes LOW.
• The transistor switches OFF, the relay disengages, and charging stops.

🔎 Visual Guide

Here’s a schematic showing the complete layout:

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🧪 Testing

• Connect your 12V lead-acid battery to the output terminal.
• Power the circuit with a 15V adapter or solar panel.
• Observe the LED: ON = Charging, OFF = Full.
• Once charged, the relay will cut the power automatically.

✅ Why This Project?

• ✔️ Protects battery from overcharging
• ✔️ Fully automatic – no manual switching
• ✔️ Works with solar or adapter input
• ✔️ Simple, modular, and upgradable
• ✔️ Ideal for UPS, inverters, and emergency battery banks

📁 Final Thoughts

This NE555-based trickle charger is a powerful little automation project for anyone managing lead-acid batteries. Whether you're running a solar setup or a DIY UPS, this build is cheap, effective, and simple to replicate.