Automatic 12V Battery Charger
This automatic 12V battery charger is designed to safely charge gel, AGM, and lead-acid 12V batteries. The charger automatically regulates the charging process and provides a LED indicator to show when the battery is fully charged.
Circuit Components
- Q1: BT136 TRIAC (requires heatsink) – controls AC power to the battery
- Q2: BRX49 SCR – regulates charging current
- T1: 12V, 4A Transformer – steps down mains voltage for charging
- F1: 3A Fuse – protects the circuit from overcurrent
Circuit Operation
- The transformer steps down the mains AC voltage to 12V AC.
- The SCR and TRIAC control the charging current to the battery, automatically reducing it as the battery reaches full charge.
- The LED indicator lights up when the battery is fully charged, signaling that the charger has switched to maintenance mode.
- The charger works safely with different 12V battery types (gel, AGM, and acid).
Key Features
- Automatic charging for 12V batteries
- Compatible with gel, AGM, and lead-acid batteries
- LED indicator for fully charged status
- High-current capability using BT136 TRIAC and BRX49 SCR
- Overcurrent protection with 3A fuse
Construction Notes
- Mount the BT136 TRIAC on a proper heatsink to handle charging current safely.
- If the charger is enclosed in a box, consider adding a small fan for ventilation.
- Use proper wire gauge to handle the charging current without overheating.
- Ensure correct polarity when connecting the battery to prevent damage.
- Fuse rating should match the transformer and battery specifications for safety.
Components
| Reference | Description |
|---|---|
| C1 | 220uF 25V Electrolytic Capacitor |
| D1 | P600 Diode – 50V 5A or greater rectifier diode |
| D2 | 1N4004 Diode |
| D3 | 5.6V Zener Diode |
| D4 | LED |
| R1, R3 | 330 Ω 1/4W Resistor |
| R2 | 100 Ω 1/4W Potentiometer |
| R4, R5 | 82 Ω 2W Resistor |
| R6 | 100 Ω 1/4W Resistor |
| R7, R8 | 82 Ω 2W Resistor |
| R9 | 1k Ω 1/4W Resistor |
| Q1 | BT136 TRIAC (requires heatsink) |
| Q2 | BRX49 SCR |
| T1 | 12V, 4A Transformer |
| F1 | 3A Fuse |
| S1 | ON-OFF Switch, > 5A at 110 or 220V |
The charger is fully automatic and can deliver up to 4 A until the battery voltage reaches a preset level. At this point, it switches to a low-current float charge to maintain the battery. If the battery voltage drops again, the charger automatically resumes charging until the voltage reaches the cutoff point. This allows the battery to remain connected indefinitely without risk of damage.
If the circuit is powered off, make sure to disconnect the battery; otherwise, it may slowly discharge.
Adjustments
R2 sets the proper charge voltage. For standard flooded and gel batteries, set around 13.8V. For AGM or GEL batteries, use 14.5–14.9V.
To adjust: connect a battery, set R2 to midpoint, power on the charger, and monitor the battery voltage with a voltmeter. Adjust the potentiometer until the LED indicator shows full charge. The charger is then correctly set.