34 12V Battery Charger ( simple and automatic)

This is a modification of 24V battery charger to cater 12V battery that requires higher charging current. Unlike the automatic 12V battery charger, this one is simpler and no comparator needed. It is more efficient also and less heating even at higher charging current. It automatically turns the charging off when the battery has reached its full charges voltage ( Vfull-charge =VDz + 1.4). The LED will light up when fully charged.
12V charger schematic
Part List :
R1 - 3.3 kilo ohms 1/4W
R2, R3 - 1.2 kilo ohms 1/4W
R4 - 5.6 kilo ohms 1/4W
R5 - see Charging Current vs R5 table below
LED - red or any 3mm or 5mm LED
Q1, Q2 - 2N2907, CS9012, 2N4403 or similar PNP transistor
Q3 - 2N2222, CS9013, 2N5551, or similar NPN transistor
Q4 - FQP27P06 or any P-channel Mosfet with higher capacity
D1 - 1N4001, 1N4004 or similar diode
Dz - 1N5242B, or any 12V zener diode
DC - 16VDC, 20A max AC-DC converter or DC power supply

P-channel Mosfet pinout
PNP transistor pinout
NPN transistor pinout

Charging Current vs R5
DC power supply
AC-DC converter schematic
Part List :
Transformer - 220V to 12V-0V-12V center tap 250VA
D - D2020L or any rectifier diode rated min 20A
C - 2200uF electrolytic capacitor rated 25V

Other useful chargers :
6V battery charger - good for 6V lead acid battery
12V charger - automatic type, current and voltage controlled
24V charger (simple and automatic) - similar to this circuit
Universal charger - charge any battery voltage form 1.2V up to 12v
Mobile phone charger - very simple charger for phones


  1. hi gyan here
    dear sir,
    i have 12v dc supply:
    i wants a circuit by which,it can charge 12v,1.2ah battery automatically and 3.7v cell phone's battery.it is requird in my project....plz help me out by sending this circuit..
    sir:becouse of this problem i m loosing my confidence plzssssss help me out.
    gyan singh

  2. Do i still need to change something here before this charger can charge my 12v 70ah battery? Pls, let me knw.

  3. ...nd what am i goin to use for R5?

  4. The only thing you are going to change is the value of R5 depending on what you need. Since battery is rated 70Ah, you need to know how long you want your battery will be charged.
    Example :battery rating 70AH and you want to charge it for 7 hrs
    Charging current = battery rating / hours of charging = 70/7 =10A
    You need 10A charging current, using the table you have R5 = 0.07 ohms rated 10W
    Meaning you can change the charging rate or time by just replacing the value of R5.
    0.07 ohms is not available, you can obtain it by just paralleling 4pcs of 0.27 ohms rated 3W each.

    Or just decide first how long you want your battery be charged and post it here, and i will design it for you.

  5. I want to charge the battery between 2-3 hours.

  6. Is there any other way to charge inverter battery(12v,100ah) automatically by inverter tranformer?

  7. Pls, give me others that i can use to replace the following; 1. 2N2907,FQP27P06,1N5242B,D2020L.

  8. For 2-3 hours charging, parallel 8 pcs 0.27 ohms rated 2W minimum. This charger can charge (2-3hr charging) but excessive heating might happen and there is a tendency that your battery will be damage.
    Using this charger, I suggest do the charging atleast 8 hours. Lower charging time, the higher the current and the higher the temperature in both the battery and the charger.
    If you really need a charger that can charge with a very short time, you need a better charger with temperature, voltage, and current control.

  9. transformer from inverter usually has the rating 12V/220V (10V/220V sometimes), meaning at a 12VAC input, the output is 220V. When we reverse it, at 220V input the output is 12V.
    12V rms has a peak voltage of 16.9VDC (with large capacitor at the output).
    Your battery is rated 12V (around 13.2V actual). Since 16.9V is much higher than 13.2V, meaning there is a chance that you can charge your battery using the transformer of the inverter. But the problem is if the actual rating of your transformer is 10V/220V or 11V/220V, you cant charge your battery. You have to rewind the secondary winding to increase its output.
    For better charging,use 220V/15V (for 220V main) or 120V/15V (for 120V main)transformer.

  10. How can i charge my inverter battery(12v, 100ah) with inverter power pack automatically. My transformer for the inverter is 220vac, 12vdc.

  11. If you are planning to salvage the transformer from your inverter, you have to rewind the secondary by additional turns so you can use it together with the charger above.
    For your original 220V/12V, make it 220V/15V

  12. pls i want to knw if i can use BT136 TRIAC in place of FQP27P06

  13. Do you think it wil not affect my inverter ac output, if i change my 220vac, 12vdc inverter transformer and i increase the secondary turn to give me 15vdc, in to charge my battery?

  14. BT136 triac for FQP27P06 Mosfet, for now i can say “no” because:
    1. Never tried building the circuit using triac.
    2. Never tried simulating using electronics simulator.
    3. Mosfet is voltage controlled and requires very small current (negligible) to drive it while triac requires much higher threshold current (still very small value but much larger than the Mosfet).
    4. The Q2 and Mosfet controls the current and replacing the Mosfet might affect its main purpose.

    However, the idea looks interesting and might work with some modifications of the circuit. I’ll try designing based on your idea if I’m not busy, and name the circuit after you.
    Thanks for asking!

  15. thanks 4 d reply,u r simply d best.once u r through with d circuit,femmy is my name.femmy automatic battery charger.LOL.....

  16. You cant use the transformer for your inverter and charger at the same time using the same battery. Changing the number or turns in your inverter winding changes the inverter output.

  17. Pls, help i dont know the different between the two terminal of the resistor, when i want to connect it to transistor or in paralel.

  18. Resistor has no designated terminals thus you can connect it in parallel whatever you want.

  19. I have built this charger. But the LED is not emitting light. Pls, help.

  20. How am i going to know if my charger is working when the LED is nt emitting light and the charge is cool?

  21. The led will light up only when its fully charged

  22. Pls, help me. LED did not light after 8hrs of charging. But i noticed that it blinks when i plug the transformer to the main supply. How am i going to know if is charging? Or what is wrong?

  23. If you observe no heat, just expect that its not charging. First you must do is make sure that the DC input is around 15V DC or 4V higher than the battery so it will charge

  24. Sir, can I replace Q4 with a relay? If no, what is the reason? how could I know the alternate IC number of Q4. I'm not getting it on the market.

  25. You cant replace it with relay but you can replace with other alternative transistor like IRF4905, IRF5305, IRF5210, IRF6218 or any P-channel mosfet with minimum current f 25A

  26. Pls sir, can I use a 4 pin or 4 diode bridge rectifier as my dc input instead of using d 2 diodes rectifier above. Is there really any difference?

  27. Hi.. i have 12v 17.2Ah sealed lead acid battery and what am i going to use for R5?

  28. Hi, would changing the zener value going to change the cut off voltage?
    Many thanks!

  29. Dear sir,

    Can you tell me is this modified shematic diagram with NMOS correct.

    Many thanks!

  30. i want to use a n -fet what change plse

    1. Open this link (http://www.2shared.com/photo/BhFBK5Ny/PUNJAC.html) and you will see the schematic diagram with NMOS, but I didn't check it.

  31. Hi, does this charger "float" charge? Thanks again, awsome job by the way.

  32. can i use this Mosfet "CD7812" in staed of FQP27P06 coz it is not possible in my area..

  33. This comment has been removed by the author.