I made a simple battery charger for charging my 12V/7Ah SLA battery commonly used inside the UPS for desktop computers. The circuit is based on the linear regulator LM317. 

The charger charges the battery in constant current and constant voltage mode. Initially, it charges in constant current mode and then moves to constant voltage mode.

How It Works ?

The input AC mains voltage is stepped down to a lower voltage ( 12-15V ) by using a step-down transformer. Then the low-voltage AC is rectified to get DC voltage by the bridge rectifier circuit consisting of 4 diodes D1, D2, D3, and D4. Then the DC voltage is filtered through the input filter capacitors C1 and C2. 

The filtered DC voltage is given to the input of regulator LM317, the out voltage from it is decided by the values of two resistors R1 and R2. This voltage is the charger setpoint to terminate the charge cycle.

Vout = 1.25 x ( 1 +R2/R1 ) 

Here I have used R1= 1K and R2 = 10K

So Vout = 1.25 x ( 1 + 10/1 ) = 1.25 x 11 = 13.75V

The output voltage from the LM317 is again filtered through the output filter capacitor C3. 

The charging current is decided by the resistor R3. I have used a 0.47Ohm / 2W resistor for setting the charging current to 500mA. However, you can reduce its value for a higher charging current. But remember not to go beyond 1.2A, otherwise, the LM317 may be damaged.

As the current through the resistor R3 goes above 500mA, the transistors Q1 and Q2 conduct, and the voltage across the resistor will increase. However, the regulator LM317 not let it happen, and the voltage slowly decreases and falls into the constant current mode.

When the charging current is low, the transistors Q1 and Q2 are in the cut-off mode, so the output voltage remains constant.

The MOSFET Q3 disconnects the charger until it is connected to the AC Mains.

The OPAMP circuit is used for LED indication when the charge cycle terminates.

Testing the Charger

To test the charger, you need a step-down transformer with a secondary voltage in the range of 12-16V. Here I have used a 220/16V transformer for the demonstration. Connect the low-voltage terminals of the transformer to the AC input screw terminal on the PCB board. 

Connect the battery with the battery screw terminal on the PCB. Be sure you are connecting to the correct polarity. The polarity is marked on the PCB. This charger has no reverse polarity protection, if you connect the battery in the wrong direction, it will be damaged.

After the connection of the transformer and battery with the charger PCB, connect the primary ( HV side ) of the transformer to the AC mains. The charger LED will immediately turn ON, it indicates the charger is charging the battery. You can check the charging voltage and current by using a multimeter. I have used my multimeter to measure the charging voltage and my clamp meter to measure the charging current. 

In the initial stage of charging, the charger current remains constant ( approx. 500mA ) but at the end of the charge cycle, it enters into the constant voltage ( 13.75V ). When the charging current is very low, the LED will turn OFF, indicating that the battery is fully charged. Now you can disconnect the battery from the AC mains.