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Fully automatic battery charger circuit using LM311

The our Fully automatic battery charger circuit  using LM311, this is a charger circuit automatic system, then when the battery voltage is lower than specified, this circuit provides power to the battery immediately and stops charging when the battery is fully only. So ideal for use in emergency power supply or battery backup power system at all type. And certainly, if you apply to your car battery, it does not have any rules.

And the most important is the price that was really very. we will quickly build is available.

*** Note: Though This project is good but it may difficut to build and expensive. I love this projects better : Simple auto cut off 12V battery charger

How this circuit works
From the circuit will be seen that, in the heart is IC1 op-amp no.LM311. Which it is connected be the compared circuit. Usually the voltage at pin 3 will be equal to 6V with the Zener diode ZD1-6V.

While the battery has a voltage charged in full. The output voltage at pin 6 to negative voltage across the Zener diode, go to make the transistor BC337-Q1 and The Relay-RY1 does not work as well. So The AC voltage input to the transformer-T1 will be cut off.


The reduction of the battery voltage will be make voltage at pin 2 of IC1 is lower than at pin 3, This makes style voltage at pin 6 of IC1 to positive and if it is higher than 3.3V, it will result in the base current (Ib of Q1) enter biased to Q1 doing and the relay also work, to contact of relay will connected the ACV to the transformer T1 so has the secondary coil ACV to a bridge circuit BD1 to DCV (15Vdc) and across drop at C1-1,000uF for filter signal full wave rectifier form, to the charging battery system again.

Until the voltage battery full will makes pin 2 of IC1 is positive when compare with voltage at pin 3, This circuit will cut off the current that charge battery at once, and begin renew charge again when voltage lower than you setup.

The component lists
IC1-LM311___Comparator Single ±15 Volt 30 Volt 8-Pin Plastic Dip Rail
Q1_____BC337___45V 0.8A NPN transistor
RY1____Relay 12V—5A contact.
C1_____1000uF 35V electrolytic capacitors
C2,C3_____0.1uF 50V Ceramic capacitors
R1,R3,R5_____22K____0.5 watts resistors
R6,R7_____1K _________”_______”
R4_____150 ohms _________”_______”
T1___Transformer 3A–12V

The creation and setting circuit
You put all components with circuit diagram shown in figure below correctly, Determine the order of the components and the polarity of the diode and the electrolytic capacitor is connected correctly or not. When done well, to try connected the power supply adjustable voltage output enter the connector for charge battery.
Note : Do not plug the AC power of this project and plug the battery.


We are setting the circuit.

Then, the voltage at 11V, and adjust VR1 until the LED light show that The relay-RY1 work properly.

The voltage increases little by little. The voltage equal to 13V, The LED will turn off that the relay to cut off the circuit. If the LED does not off to adjust VR1 again. The adjustment VR1, which may be adjusted to adjust several times, to the circuit as appropriate.

This is other circuit idea use LM311 for charger battery.

Auto Fast Battery Charger by LM311 for CAR

This is Circuit Auto Fast Battery Charger.Here is circuit Auto Charger battery 12V will quickly and easily charge most any lead acid battery. When the battery is fully charged, the circuit Relay switches off
Use IC 7805 for power supply here circuit. and IC LM311 control test volt battery .
Read detail in circuit.



Many friends say that this builds a difficulty. If you want to build a simple circuit, I suggest below :The most lead-acid battery charger circuit by LM317

Other circuits are recommended.

  1. Automatic ni-mh battery charger can make by yourself
  2. Automatic Battery Charger 6V-12V with Relay cut off by LM324
  3. Automatic OFF 12V Battery Charger by power SCR

*** Note: These circuit below are little IDea for compare voltage in the auto battery charger.
Now this circuit not works well fully 100% (relay oscillation).
Thus if you make this circuit you must know more electronics.***

This Post Has 29 Comments

  1. Please be careful in grammatical mistakes in your circuit description format.

  2. Want to purchase a complete kit of battery charger or pcb. Plz. help by sending a price list for the product and others.


  3. This diagram is nonfunctional. The pin numbering for LM311 is wrong. Check with the original datasheet from NS or ON-SEMI, for example. R4 and R5 are useless. A certain amount of positive feedback(hysteresis) should be employed to prevent chattering. And tie pin 4 to GND(-). Lower R2 to 15kΩ.

  4. I have tested out this circuit. It works well. PLEASE NOTE THAT IN THE CIRCUIT DIAGRAM, THE OUTPUT OF THE COMPARATOR IS GIVEN AS 6. THE ACTUAL PIN OUT IS 7 which is a slight mistake. I tried it out in breadb.. Now going to set it as my charger for inverter just constructed.

    Thank you for sharing the circuit..:)

  5. Dear Guys,

    Sorry for my previous update.

    The circut has some disadvantages. The relay starts vibrating at the trigger volage adjusted with the preset resistor. So when the battery just gets full, the relay starts vibrating.

    The circuit cannot be used in live case. It is just a circuit idea.


  6. How do I connect my circuit board so that I can power up my 500W inverter and 12V DC light simultaneously using a 12V rechargable battery?

    Any tips?

  7. 12v Battery charging circuit and cut off automatically while battery was low at 8v when the charging was on

    Plz i want this circuit

  8. thanks 4 dat share guy.
    Wot if i want to use it 4 a 24volt battery,wot component wil i charge.
    Also aw do i know d value of zener diode to use,if any calculation is involve pls let me know

    And again is d comparator pin6 or 7

  9. you have display all and different kinds of power supplies, nice and it definitely helps me a lot..but i need only simple automatic cutoff battery charger with less spare parts used..
    thank you..

    hope you can add tye simplest diagram i need,

  10. can anyone provide me info about the working of potentiometer and zener diode in this ckt…i’ve made it but the auto cut off part is not working…the voltage across zener diode is coming .73V…do i have to change potentiometer resistance????
    pls help asap!

  11. Hi guys,
    i have made same target with IC 555. It is working well for more than 2 years. Its main advantage is that chattering of relay is totaly eliminated. Low and full charge is set by 2 different pot easily. Also the cost of ckt is same as this ckt. Mail me for more info:
    [email protected]

  12. why my relay is chattering when my battry is full charged.pls help

  13. In LM331 6 pin is not used as output. This is mistakenly printed. You must have to correct it. Actually output is taken from pin 7.

  14. This charger has not an adjustable hysteresis threshold, because there is missing the hysteresis resistor with a trim pot in serial. Buy at Freight Harbor Tools a Battery Float Charger, Automatic on sale for $9.99 or ($6.00 with coupon). The factory 13.2V is only a promise. Open the regulator, replace the 56 ohm resistor marked as VR1, with a 100 Ohm trim pot via two tiny wires. Now you have a Voltage regulated battery trickle, from 12.5 V to 14.25 V. Float charging a battery is like dancing on a needle. At 11.4 v the car battery is discharged and at 12.9V is fully charged. Increasing the voltage above 2.26V per element or > 13.5V the battery start boiling. Tiny current as low as 20mA, for long period of time oxidize the positive elements. I set the regulator at 13.25V on the car and forget. I have more than one car and while not in drive I trickle the battery to keep at 12.9 V without charging element. This is only the first AID. Periodically you have to equalize the elements of Battery, This can be done increasing the charging voltage, periodically, to 14.5 V for 2 hours. While the weaker cells are charging the strongest cell are boiling. The goal is to equalize the voltage 2.26V per each cell. Periodically check electrolyte level and top it with distilled water on mark. If decide to maintain the battery, do not buy sealed maintenance free batteries. To be able to equalize you need a Battery tender with Hysteresis Threshold, that start charging at 12.6V and stop charging at 14.5, giving the battery a chance to rest or relax.

  15. Year that’s simple circuit for use,@Presoon John says : when the battery just gets full, the relay starts vibrating. the C3 capacitor can chan to 0.01uf, or try to as low C1 to 100~220uf (that’s mean time to hold low) – it’s look so good

  16. Hi ALL,

    If anyone require a best battery charger with both low and high set point mail me at [email protected] this charger is simple to build and takes only few milli ampere current from battery. very sophisticated design.

  17. i wont taken it’s items how can take

  18. Why when i try to print an article it is to big for the paper i am printing on?? Tried it on a couple different computers and printers…Have the fit ti page box checked and the article is printing widwe than the paper???

  19. How i connect another 12v spare battery to my car? i mean both the battery using one by one when need. Can any one ans my qstn?

  20. Could someone please inform how to prevent Relay chatter

  21. How to prevent Relay chatter ? Could someone please explain

  22. can i change ZD1 with 5.6V ???

  23. Hi,

    I was hoping to build your battery charger using LM311 design but quickly realized the component list was incomplete (VR1 10K) for example 🙁

    And trying to decipher the poorly illustrated component schematics is a pain for such a simple layout…what’s 4001 & 6U for example? 🙁

  24. don’t trues in this circuits has many mistakes

  25. Relay Chatter should be a problem here as Mr Tharanga pointed out.. Theoretically a hysteresis is to be added. I am yet to see a reasonably simple circuit to achieve that, can someone show here?

  26. I set up the circuit with all the mentioned components, and it really works well with the variable power supply, but when we use the battery, the relay switches constantly. Can someone explain why it happens and how do we get around this problem?

  27. sir sercuit is working but realy is continus on off sound give now what will do

  28. Sir how Lm 311 works as op amp
    Is the lm 317 is work as opamp?

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