auto cut off 12v Battery Charger
Design an auto cut off 12V battery charger using BC547 transistor for safe charging. Automatically disconnects when battery reaches full charge.
automatic 12V battery cutoff charger
A battery charger with an auto cut-off 12V battery charger that uses BC547 disconnects and automatically stops charging once the battery has been charged to full voltage, and eliminates the risk of overcharging the battery.
transistor-based auto cutoff circuit
Auto Cut Off 12V Battery Charger with BC547 transistor is a low-cost and easy-to-use charging system that can be used to charge 12V lead-acid or lithium-based batteries. The primary operation of this circuit is to automatically discontinue charging the battery when it is fully charged to prevent overcharging the battery and increase the battery's lifetime. The NPN transistor device, known as the BC547, is used as a switching device with a preset voltage divider network and a resistor. This is why, when the battery voltage is lower than the preset value, the transistor will conduct to charge the battery. The voltage is turned off as soon as the voltage reaches the full charge (approximately 14.4V when using lead-acid batteries). This renders the circuit energy efficient, safe, and cost-effective. It is simple to assemble on a breadboard or PCB and only requires a small number of components (BC547, resistors, zener diode, relay). The circuit finds application in homemade chargers, solar chargers, and UPS back-up chargers.
Work / Installation (Inputs → Outputs)
- AC/DC adapter input provides 15–18V supply.
- Rectifier & filter deliver smooth DC for charging.
- BC547 transistor + relay control charging path.
- Battery output receives charging until cutoff voltage is reached.
Testing & Final Adjustments
Start with a test with a 12 V lead-acid battery. Tune the preset potentiometer by making the relay turn off at 14.4V (full charge). Measure the voltage of the battery at the time of charging using a multimeter. Make sure that the relay does not activate and deactivate. In case of premature cutoff, trim the zener diode or preset value. Test: Discharge and recharge the battery several times to ensure the battery's operation is reliable. The transistor BC547 can be cooled under low conditions, but a transistor or relay driver is needed with high current chargers. Lastly, lock the connections and make sure that they are insulated so that they do not short.
Frequently Asked Questions - auto cut off 12v Battery Charger:
What is the role of BC547 in this charger?
It acts as a switch to control charging cutoff.
At what voltage does it cut off?
Around 14.4V for a 12V lead-acid battery.
Can it charge lithium batteries?
Yes, but voltage cutoff must be adjusted accordingly.
Is a relay required?
Yes, to disconnect charging path automatically.
What is the input voltage needed?
Typically 15V–18V DC supply.
Can it be used in solar charging?
Yes, it works with solar panels as well.
Does it prevent overcharging?
Yes, it disconnects automatically when full voltage is reached.
Is it suitable for UPS batteries?
Yes, ideal for 12V lead-acid UPS batteries.
Is it easy to build?
Yes, it requires only a few components.
Does it need heat sink?
No, BC547 handles small current without heating.
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