🔋 DIY Auto Cut-Off 12V Battery Trickle Charger Using NE555 IC – Easy Homemade Project!

Are you tired of overcharging your 12V battery and reducing its lifespan? Want a smart solution that safely charges your battery and cuts off automatically once it’s full?

In this step-by-step Instructable, you’ll learn how to build a fully automatic 12V battery trickle charger with an auto cut-off feature at home using the popular NE555 timer IC. It’s simple, efficient, and perfect for lead-acid battery maintenance.

Let’s dive into the full guide, circuit, components, and working!

Required Components

1. NE555 Timer IC – 1 piece
2. 1N507 Diode – 1 piece
3. 1NB62 Zener Diode (6.2V) – 1 piece
4. D36 Diode – 1 piece
5. 10µF Electrolytic Capacitor (C22) – 1 piece
6. Resistor 1kΩ – 1 piece
7. Resistor 62Ω – 1 piece
8. Resistor 4.7kΩ – 1 piece
9. Resistor 10kΩ – 1 piece
10. Resistor 12kΩ – 1 piece
11. General Purpose NPN Transistor (e.g., BC547) – 1 piece
12. SPDT 12V Relay (e.g., 650-1) – 1 piece
13. Dual LED Indicator – 1 piece
14. Diode for relay protection (optional) – 1 piece
15. Terminal Blocks or Wires – as required
16. Power Source (15V DC adapter or solar panel) – 1 piece
17. Heat Shrink Tubes/Enclosure (optional for safety)

1. How to use NE555 as a voltage level detector
2. How to control a relay for auto cut-off
3. How to make a safe trickle charger for 12V batteries
4. Protect your battery from overcharging

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The above circuit shows how the NE555 timer is used to sense the battery voltage and trigger a relay for automatic cut-off when the battery is full.

🧠 Circuit Diagram Explanation

🧩 1. Power Input & Protection Section

1. The circuit is powered by a 15V DC source (could be a wall adapter or solar panel).
2. Diode 1N507 and Zener Diode 1NB62 (6.2V) protect the circuit from reverse polarity and overvoltage.
3. D36 provides grounding and ensures current flows correctly.

🧩 2. NE555 Comparator Configuration

1. NE555 Timer IC is configured as a comparator.
2. The voltage divider (R2, R4) senses the battery voltage.
3. When battery voltage is low, the voltage at pin 2 (TR – Trigger) is below 1/3 VCC, making the output at pin 3 HIGH.
4. This turns ON the transistor and activates the relay.

🧩 3. Relay Control Mechanism

1. The output of NE555 (pin 3) goes to the base of an NPN transistor (via 1kΩ resistor).
2. When the transistor turns ON, it completes the circuit for the relay coil.
3. Relay energizes and connects the charging voltage to the battery.

🧩 4. Auto Cut-Off Feature

1. As the battery charges, its voltage increases.
2. When it reaches the full charge level (~13.7–14V), the voltage at pin 2 goes above 1/3 VCC.
3. This makes the NE555 output go LOW, turning OFF the transistor and deactivating the relay.
4. Charging is cut off automatically, preventing overcharging.

🧩 5. LED Indicator

1. A dual LED is connected to indicate charging and cut-off states.
2. When the battery is charging, the LED glows.
3. When fully charged, the LED turns OFF or changes color (depending on wiring).

🔧 Step 1: Build the Voltage Divider and Zener Network

1. Connect R2 and R4 in a voltage divider configuration to pin 2 of NE555.
2. Add the Zener diode between VCC and GND to set a stable reference voltage.

🔧 Step 2: Connect NE555 Circuit

1. Wire pins:
2. Pin 1 – GND
3. Pin 2 – Voltage from divider
4. Pin 3 – Output to transistor
5. Pin 4 – VCC
6. Pin 5 – CV (connect to GND through 10k resistor)
7. Pin 6 and 2 – Shorted together
8. Pin 7 – DIS (not used here)
9. Pin 8 – VCC

🔧 Step 3: Add Relay and Transistor

1. Connect a 1kΩ resistor between NE555 output and transistor base.
2. Connect transistor emitter to GND.
3. Connect relay coil between VCC and collector.
4. Place a freewheeling diode across the relay coil (not shown but highly recommended).

🔧 Step 4: Connect the Output Section

1. Connect the battery to the relay’s NO (Normally Open) terminal.
2. Connect charging input to relay’s COM (Common).
3. Connect LED across relay terminals (through a resistor) for indicator.

Connect a discharged 12V battery to the output terminals.

Power the circuit using a 15V adapter or solar panel.

The relay should click ON, and the LED should light up – battery is charging.

Once the battery voltage reaches ~13.8V, the relay turns OFF – charging stops.

⚡ Auto cut-off when battery is full

🔋 Prevents overcharging and battery damage

🛠️ Simple to build using common components

💡 LED indicator for charging status

☀️ Can be powered by solar panels or adapters

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This Auto Cut-Off 12V Battery Charger is an excellent project for beginners and hobbyists. It helps keep your batteries healthy, saves energy, and adds automation to your DIY toolkit.