12V Battery Charger-cum-Variable Power Supply

Fayaz Hassan


Construction and testing

A single-side PCB for the test points5812V battery charger-cum-variable power supply is shown in Fig. 2 and its component layout in Fig. 3.

After assembling the circuit on the PCB, place it in a suitable cabinet. Fit CON3 and CON4 on the front side of the cabinet. CON3 is for variable power supply (VPS*) and CON4 for battery under charging (BUC#).

Connect CON3 between N/C2 terminal of relay RL2 and point 4 given on the PCB. Connect CON4 between points 3 and 5 given on the PCB. Fix VR1 through VR5 and all LEDs on front side of the cabinet, and label these as shown in the circuit diagram.

Place transformer X1 inside the cabinet. Fix CON1 and switch S1 on the rear side of the cabinet. Use proper heat-sinks for IC1, IC2 and IC4, and connect a 12V cooling fan at CON2 for fast cooling.

The circuit works off 230V mains power supply. Refer the test points table for checking the voltages at various points before using the circuit.

Initial setup and testing

  1. Remove jumper J1 and J3, connect J2 and switch on S1.
  2. Adjust VR1 and VR2 to get 9V DC (as battery dead voltage or as required) with respect to ground at TP6.
  3. Adjust VR4 such that you can turn on and off status LED2 and LED6 alternately. RL1 and RL2 also change their states (energise/de-energise).
  4. Adjust VR2 to get the full battery voltage (say, 13.4V DC with respect to ground) at TP6.
  5.  Adjust VR5 to turn on LED4, if TP7 is connected to ground through J3.
  6. Once LED4 is set, adjust VR2 to get battery charging voltage (say, 14.2V DC with respect to ground) at TP6.
  7. Connect J1 (J2 still connected) and adjust VR3 to get battery standby voltage (say, 13.4V DC with respect to ground) at TP6.
  8. Once the required voltages are set, remove J2. The circuit is ready to use.
  9. If no battery is connected at CON4, variable voltage is obtained at CON3 by using VR1. Brightness of LED7 is proportional to the output voltage at CON3.
  10. If 12V (>=50Ah) battery is connected at CON4 in reverse polarity, buzzer PZ1 gives an alarm sound and LED5 glows.
  11. If the same battery is connected in correct polarity at CON4, status of the battery is indicated by LED2 (healthy) or LED6 (dead). RL1 and RL2 will get energised in case the battery is healthy and the battery will get charging voltage. Once the battery is fully charged, LED4 glows, which indicates that the battery is fully charged and the charger is at standby voltage.
  12. Use proper crocodile clips for connecting the battery terminals.

EFY note

1. Charging voltage, standby voltage and battery dead voltage may vary as per the manufacturer of the battery. The values indicated here are safe voltage values.

2. Heat-sink with mica insulation must be provided for IC1 and IC2.

Download the PCB and component layout PDFs: click here

Fayaz Hassan is a manager at Visakhapatnam Steel Plant, Visakhapatnam, Andhra Pradesh. He is interested in microcontroller projects, mechatronics and robotics



  1. Thanks Mr. Fayyaz Hassan. Great work that you have brought through this article. May we discuss further if you are interested to be a part of our upcoming project as virtual guide for technical assistance. Please revert back to Mob. No. 9767720880 or report to [email protected].

  2. Thank You So much for the circuit.
    But I can’t find a position for relay Rl2 on pcb. Its not listed in pcb diagram. Kindly guide through it.

  3. During initial setup of circuit, should the battery be disconnected or connected when setting charge, dead, float voltages etc?

  4. Hello
    I need an 18v auto charger
    If you send an orbit, I will be very grateful
    Is this 12 volt circuit up to a 18 volt charger? Which part should be replaced?
    How should i make it
    Thank you
    I am waiting for your mail

  5. the same as the question asked by our friend Mr. Usama Naveed. During initial setup of circuit, should the battery be disconnected or connected when setting charge, dead, float voltages etc?

  6. Do the transformers available in local market rated 5 Ampere output the full 5 A current or only half the current ?
    As it’s said that no electrical or electronic device can deliver full 100 percent power, so I want to know whether any transformer available has 100 % efficiency or only 50 % ?


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