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Self Regulating Battery Charger Circuit

Last Updated on May 10, 2021 by Swagatam 261 Comments

The post explains how a neat little self regulating automatic battery charger circuit can be made using just two inexpensive transistors.

This circuit will automatically regulate the charging supply to the battery depending on its charge level, by switching the input supply ON and OFF periodically.

How it Works

As can be seen in the diagram, this auto-regulating battery charger circuit utilizes just two transistors for detecting the charging thresholds, and cuts off the process as soon as these limits are detected.

Using two transistor actually makes the design hugely sensitive compared to a single transistor charger circuit.

The indicated preset is set in such a way that the T1 is just able to conduct at the specified full charge threshold of the battery.

When this happens T2 begins switching OFF, and ultimately at a point it is unable to sustain the relay conduction and switches OFF the relay, which in turn cuts of the input charging source with the connected battery.

Conversely, when the battery voltage begins dropping, T1 gradually deprived of its adequate conduction voltage level, and ultimately it ceases to conduct, which quickly prompts T2 to initiate its conduction and trigger the relay into action,

The relay now reconnects the charging input supply with the battery, and restores the charging process until it yet again reaches the full charge threshold, when the regulating cycle repeats itself.

How to Sep Up the Circuit

Setting up this battery charger circuit for automatic regulation is very simple and may be done in the following way:

  • Initially, do not connect the fixed transformer power supply; instead connect a 0-24V, variable supply voltage to the circuit.
  • Remove the anode of D6 from the relay contact and connect it to the positive of the power supply.
  • Keep both the presets somewhere at the center position.
  • Switch ON the power and adjust the voltage to 11.5 volts or lower.
  • Adjust P2, so that the relay just activates.
  • Now increase the volts to about 13.5 volts, and adjust P1 so that the relay just deactivates.

The setting procedure of the circuit is now complete.

Check the whole procedure by continuously varying the voltage up and down.

You may now remove the variable power supply and connect the fixed transformer, bridge power supply to it.

DON’T FORGET TO RECONNECT THE ANODE OF D6 BACK TO THE RELAY CONTACT OR THE BATTERY POSITIVE.

The battery connected to this circuit will be charged only as long as its voltage is in between the above "window" level.

If the battery voltage crosses the above "window", the relay will trip and stop the battery from charging.

Parts List

  • R1, R2 = 10K,
  • P1, P2 = 10K PRESET,
  • T1, T2 = BC 547B,
  • C1 = 2200uF/25V
  • C2 = 47uF/25V (Please connect this capacitor across the relay coil)
  • D1---D4 = 1N5408,
  • D5, D6 = 1N4007,
  • RELAY = 12 VOLT, SPDT,
  • TRANSFORMER = AS PER THE CONNECTED BATTERY AH (DIVIDE BY 5)
self adjusting battery charger circuit
caution electricity can be dangerous

The following diagram shows the instructions which needs to be followed while setting-up the circuit with the desired cut-of thresholds, using a variable power supply unit:

setting up a self adjusting battery charger circuit

The above self-regulating battery charger circuit was successfully built and tested by Mr. Sai Srinivas, who is just a school kid but nevertheless has an immense interest in the field of electronics.

The following images were sent by him which displays his talent and intense dedication in the field.

#1 self regulating battery charger prototype images
#2 self regulating battery charger PCB images
#3 self regulating battery charger LED control
#4 self regulating battery charger prototype working

For One Shot Operation

If you want the above circuit to lock itself into a permanent cut off position when the battery is fully charged, then you may modify the design as shown below:

one shot battery charger over charge protection

Note: To ensure the relay does not latch itself quickly on power switch ON, always connect the discharged battery first across the shown terminals and then switch ON the input power.

In order to indicate the charging status of the battery, we can add a couple of LEDs to the above design, as shown below.

The above circuit was also successfully built and tested by one of the dedicated electronic enthusiasts from this blog. The following pictures verify the results:

Battery is charging, as indicated by the orange LED
Battery charger is set to cut off at 12.92V, therefore both the LEDs are ON now, indicating the cut-off threshold has reached
Charging is now cut-off, as indicated by the RED LED ON, and Orange LED switched OFF, at the set 12.92 V
Image showing the Battery that was used for the charging process

Warning: Make sure to test the preset setting thoroughly before you leave this application unattended. And be sure to use only 14V as the input supply for a 12V battery.

You'll also like:

  • 1.  Single Transformer Inverter/Charger Circuit
  • 2.  Bicycle Dynamo Battery Charger Circuit
  • 3.  3 Step Automatic Battery Charger/Controller Circuit
  • 4.  Battery Backup Time Indicator Circuit
  • 5.  Make a Battery Charger in 15 Minutes
  • 6.  12 V to 19 V Converter Circuit

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

Have Questions? Please Comment below to Solve your Queries! Comments must be Related to the above Topic!!

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