Feb 182020

Auto shutdown 12V battery charger circuit using op-amp & voltage regulator

This 12V battery charger auto shutdown circuit allows you to charge a 12V car battery (lead acid battery) using a voltage regulator and an operational amplifier. This circuit is very useful when the battery is discharged and we cannot start the car.

Operation of the 12V Battery Charger Auto Shutdown

The charger will charge the battery at maximum current (limited to 2 amps) when the battery is discharged, and will gradually reduce the current as the battery voltage approaches its maximum voltage. When this happens, the current is limited to approximately 150mA.

When the “Start” button is pressed (to start the charging process), the voltage delivered by the charger is 14 volts.

12V Battery Charger Auto Shutdown Circuit using op-amp & voltage regulator

12V Battery Charger Auto Shutdown Circuit using op-amp & voltage regulator

At this point, the operational amplifier, which acts as a comparator, has the current battery voltage at its inverting input and the reference voltage at its non-inverting input. Thus, if the battery is not fully charged, the output will keep transistor Q1 in its cutoff region.

As the battery charges and requires less power from the charger, the voltage is reduced to 12.5 volts and the charging process is completed.

When the battery is fully charged, the comparator has a low voltage output, transistor Q1 goes to its saturation region, and the LED lights to indicate that the battery charge is complete.

Parts List for the Battery Charger Circuit

  • U1: LM350 voltage regulator
  • U2: LM301A operational amplifier (op-amp)
  • C1: 0.1uF capacitor
  • C2: 0.1uF capacitor
  • C3: 1000pF capacitor
  • R: 500 ohm resistor
  • R1: 3K, 1/4 watt resistor
  • R2: 15 ohm, 1/4 watt resistor
  • R3: 230 ohm, 1/4 watt resistor
  • R4: 15K, 1/4 watt resistor
  • R5: 1K, 1/4 watt resistor
  • R6: 0.2 ohm resistor, 2 watts or more
  • D1: 1N467 rectifier diode
  • LED: 1 green
  • Q1: 2N2905 PNP bipolar transistor
  • Other: 1 momentary switch, normally open

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