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.***


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