Automatic battery charger

This is a project of an automatic battery charger circuit, which interrupts charging when the battery reaches a limit.

How the automatic battery charger works?

It’s possible to charge a battery using only a voltage source, a resistor, whose function is to limit current, and a diode, to prevent current flux in the opposite direction.

Therefore, it’s necessary to have a voltage sensor, which indicates if the battery reached a determined value, if yes, the charging must be interrupted to avoid overcharging the battery. The battery’s charging status must be indicated by LEDs.

Material list for automatic battery charger

Printed circuit board.
Wires.
Transformer, whose secondary is 12V.
4 1N4007 diodes.
Electrolytic capacitor 470μF x 16V.
Zener diodes: 1N4737 and 1N4736A.
UA741CN.
Resistors: 1 of 10kΩ and 3 of 1kΩ.
NPN transistor TIP122.
A 12V relay.
LEDs: a red and a green.

DC power source

automatic battery charger source — This automatic battery charger was designed to be connected to a power socket. A transformer decreases the voltage, a diode bridge rectifies alternate current signal and an electrolytic capacitor converts the rectified pulsating signal in a continuous signal with a ripple.

Voltage sensor and relay

In this project, to indicate that the voltage reached a limit, the charging battery must also be linked to the inverting input of an operational amplifier (op-amp), it uses the UA741CN, whose datasheet is on this link. While on the non-inverter input, there is a 1N4737 zener diode and the R1 resistor, they maintain a constant voltage on non-inverting input. Op-amp serves as a comparator, when the battery’s voltage reaches approximately 7V, op-amp’s output emits a signal to transistor’s base, activating the relay, by it’s turn, makes the commutation, interrupting and preventing overcharging.

The voltage sensor schematic. While BT1 voltage is lower than approximately 7V, the red LED turns on, if it exceeds this voltage, the red LED will turn off, while the green LED will turn on.

These are UA741 pins, whose source is the chip’s datasheet. Therefore, you can link 7 and 4 terminals to the positive and negative poles of a DC source, respectively.

I chose the Tianbo HJR-3FF-S-Z relay, whose datasheet link is below the image. 5 and 2 are coil’s terminals, which control the mobile contact (pin 1).