26 Automatic 12V Battery Charger

This time we are going to build an automatic charger for  12V lead acid battery. First we need to know the guidelines of a good charger for a lead acid battery and this guidelines will be our basis in our design.

According to sources  a good charging consist of three stages namely the constant current charging stage, followed by constant voltage charging and the final stage which is the float charge.

The scenario is that initially the discharged battery is charge at a constant current of about 10% to 30% of its rated capacity until it reaches to a certain voltage (around 2.3V per cell) it begins to saturate. Then this time the constant current charging is turn off and the charger employ a constant voltage charging until the current delivers to the battery drops down about 3% of its rated value.After reaching this point, the charger starts the float charging which is to compensate battery self discharge.

The diagram below shows the the three stages together with the state of voltage (yellow) and current (red). Click to enlarge.
Charging stages of Lead Acid Battery
Our Battery Charger design:
1. Meet the good charging guidelines of lead acid battery
2. Battery charger should be simple and cheap.

Charger Schematic and Operation Click to enlarge
charger circuit 
From the circuit above, the charger is composed of AC to DC converter, comparator for constant current and constant voltage control, transistors and zener diodes.
If the discharged battery has an open voltage of less than 13V, the output of  comparator U1 is open while U2 is shorted to the ground. In this case, constant current charging is implemented and controlled only by the
R3 and zener2 combination with magnitude of current to be delivered to the battery is :
Current (I) =( (Zener2 voltage)-0.7V) / R3

When the voltage of the battery reaches over 13V, U1 is shorted to ground while U2 is open. In this case, constant voltage charging is performed. The magnitude of voltage is limited and controlled only by Zener3.
Battery Voltage =  (Zener3 voltage) -1.4V

You can modify and improve the circuit by adding LED indicators and change the magnitude both voltage and current by simple using the formulas above.

Transformer - 220V to 15V-0-15V, 15 to 20VA
R1 - 50K-ohm 1/4W resistor
R2 - 5K-ohm,1/4W resistor
R3 - 3.3-ohm 5W resistor
R4 - 3.9K-ohm 1/4W resistor
zener1 - 13V 1/2W zener diode (1N5243B)
zener2 - 4.7V 1/2W zener diode (1N5230B)
zener3 - 15V 1/2W zener diode (1N4744A)
Q1,Q3 and Q4 - NPN general purpose transistor (9013,2n222, etch)
Q2 - PNP power transistor (TIP32C or TIP42C)  pin out below
C1 - 1000uF 25V electrolytic capacitor
D - 1N4001 or any rectifier diode
U1 and U2 - dual comparator (LM393)  pin out below

TIP32 and TIP42 Pinout
LM 393 pinout


  1. This circuit is simple and efficient.
    I'm planning to add a battery monitor circuit using LM3914 with this circuit in order to make the status of the battery.

  2. What is its maxium charging current?
    How do I get it to charge with a current of 4A?

  3. Both SLA and AGM batteries have different charge cycle treatment, does this circuit account for that or it just for the Valve regulated lead acid battery?

  4. Does this circuit safe guard against discharging the battery in the event of a blackout?

  5. Good day pipZ!

    If you need around 4A current, use the formula below using available zener diode and resistor.

    Current (I) =( (Zener2 voltage)-0.7V) / R3

    In our circuit since we got a zener diode (4.7v forward voltage) you can use resistor of about 1 ohm to get 4A current

    This simple charger is designed to charge primarily LEAD acid batteries. Its charging process is very simple and straightforward . We dont recommend to charge Batteries that require special or specific charging pattern or cycle.

    This simple charger protects battery form discharging during brownouts

  6. does this circuit stops automatically if the battery is already full?? can i use this to charge 8 1.5V battery??

  7. does this circuit stops automatically if the battery is already full?? can i use this to charge 8 1.5V battery??
    Charger automatically reduce the current to a very minimal amount when fully charged to compensate battery self discharging.
    You cannot use 8 X 1.5V battery and i don't recommend it because this charger is designed for lead acid batteries. Other types of batteries requires different charging cycle and has different charging current and voltage.

  8. Can i use it to charge 12v 70ah battery?

  9. Of course you can charge 70AH battery. But you must replace the following parts.

    Q2 - MJE2955T or higher power PNP transistor
    Z2 - 2.7V zener minimum 1/2W
    R3 - parallel seven pcs 2 ohms (2W)

    though this can charge the battery, excessive heating on parts will be observed. The circuit becomes inefficient when charging this 70AH battery. Maybe ill design a charger for you to cater such large battery. For the meantime i suggest to modify the 24V battery charger since its good for such large battery.

  10. I will be very happy if you can do that for me. Thank you

  11. Is color code show the the wattage of resistor? E.g 3.3 kilo ohms 1/4watt.

  12. Ordinary resistor color code shows only the resistance and tolerance, but no wattage. But wattage value is usually marked in the body of the resistor
    FYI only! when we say 3.3 kilo ohms 1/4W, it means you can use resistor with resistance 3.3 kilo ohms and power rating 1/4W minimum. Meaning you can use any power rating higher than 1/4W of the same resistance

  13. Here you have written that

    "initially the discharged battery is charge at a constant current of about 10% to 30% of its rated capacity until it reaches to a certain voltage (around 2.3V per cell) it begins to saturate."

    my understanding from this statment is that:

    for a completely discharged 7AH, 12V sealed lead acid battery; should be charged at a range of:

    10% of 7A = 0.7A
    30% 0f 7A = 2.1A

    charging current should not more than this, else it will damage the battery.

    am i ritght??

    or i can infeed higher currents up to 5A to charge the battery for its fast charging.
    Please Guide me about that.
    thank you

  14. Also
    what should the output of a charger to charge the bettery.
    suppose i have 12V sealed lead acid bettery, so what should be the voltages at the output terminals of the charger to charge the bettery. i wana ask about the specific charger to charge a 12V bettery.

    secondly if a simple bridge rectifier is taken and a capacitor of 3300 or 2200uF is connected to it. will it charge the bettery? will it decrease the current flow if the bettery is charged and stop delivering current when it get fully charged? i mean automatically.

  15. How can this be modified to charge a 6v 12 Ah SLA? This sounds like exactly what I need but don't know what has to be changed.

  16. I have try to modify this circuit to Charge my 6V 2.2Ah Sla by follow:
    R1 47K,R2 4,7K,R3 9R(2x18R paralel),R4 3.9K,
    zener1-6.8V,zener2-2,7V,zener 3 8.8V(ZD8.2V+0.6V 1n4148)=ca. 8.8V!

  17. Respected poster of the circuit,

    Could you please explain where an LED should be connected to indicate full battery charge? Thank you very much!

  18. Hi there,

    I'm in the process of modifying this to use as a 24V charger. The only question I have is about the comparators. Are the inputs the wrong way around on the schematic? As it's drawn, it looks like U1 output = Vs when Vbattery > Vzener and for U2 Vout = Vs when Vbattery < Vzener

  19. Dan is right, your schematic will give an output where Constant voltage charging mode will occur first before entering constant current charging mode. It should be the other way around. This is because of the wrong comparator pin assignment.

    Pin 3 and 6 should connect to the 13V zener diode whereas
    Pin 2 and 5 should connect to the 12V lead acid batteries.

  20. I take back what I said, the circuit is connected correctly. Mistaken with the concept of comparators. When positive pin is greater than negative pin, no current will flow at output, negative>positive pin then current will flow at output

  21. Hola, ¿puede dejarse la batería conectada siempre, para que se cargue y descargue automáticamente? Es para usarla con un circuito de alarma. Saludos...

  22. Hello, you can be left always connected to the battery, so that it loads and automatically download? It is for use with an alarm circuit. Regards...