singletransistorautobatterychargerwithcutoff - Single Transistor High Current Battery Charger Circuit

Single Transistor High Current Battery Charger Circuit

The post discusses a single transistor high current battery charger circuit with an automatic over charge cut off feature. The idea was suggested by Mr. Raja Gilse.

The Circuit Request

Dear sir, Swagatam
I have a 2N6292 . My friend suggest me to make the simple fixed voltage high current DC power supply to charge the 12 v battery. He had given the attached rough diagram. I don't know anything about the above transistor. Is it so ? My input is 18 volt 5 Amp transformer. He told me to add 2200 uF 50 Volt capacitor after rectification. Is it works ? If so , is there any heat sink necessary for transistor or/and IC 7815 ? Is it stops automatically after battery reaches 14.5 volt ?
Or any other alteration needed ? Please guide me sir

Solving the Circuit Request

Yes it will work and will stop charging the battery when around 14 V is reached across the battery terminals.

 However I am not sure about the 1 ohm base resistor needs to be calculated correctly.

The transistor and the IC both may be mounted on a common heatsink using mica separator kit. This will exploit the thermal protection feature of the IC and will help safeguard both the devices from overheating.

single transistor high current battery charger circuit

Circuit description

The shown single transistor high current battery charger circuit is a smart way of charging a battery and also achieving an auto shut off when the battery attains a full charge level.

The circuit is actually a simple common collector transistor stage using the shown 2N6292 power device.

The  configuration is also referred as an emitter follower and as the name suggests the emitter follows the base voltage and allows the transistor to conduct only as long as the emitter potential is 0.7V lower that the applied base potential.

In the shown single transistor high current battery charger circuit, the base of the transistor is fed with a regulated 15 V from the IC 7815, which ensures a potential difference of about 15 - 0.7 = 14.3 V across the emitter/ground of the transistor.

The diode is not required and must be removed from the base of the transistor in order to prevent an unnecessary drop of an extra 0.7 V.

The above voltage also becomes the charging voltage for the connected battery across these terminals.

While the battery charges and its terminal voltage continues to be below the 14.3 V mark, the transistor base voltage keeps conducting and supplying the required charging voltage to the battery.

However as soon as the battery begins attaining the full and above 14.3 V charge, the base is inhibited from a 0.7 V drop across its emitter which forces the transistor to stop conducting and the charging voltage is cut off to the battery for the time being, as soon as the battery level begins going below the 14.3 V mark, the transistor is switched ON again...the cycle keeps repeating ensuring a safe charging fr the connected battery.

Base resistor = Hfe x battery internal resistance

20 thoughts on “Single Transistor High Current Battery Charger Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hi Sir good day to you, id like to ask a favor if you can give me a citcuit
    for low and high cut off switch for my battery. The scenario is this.

    My battery is being charged by rewired washing machine Fisher and Paykel motor (Generator),
    to give you a better birds eye view of that matter pls see link below:

    Based from that link i dont have that water power source there in my place, so what i did
    is instead of water to drive the generator i used a 6WATT 100 RPM AC gear motor to
    spin the genarator thru a compound gear to gain more RPM.

    My problem is i want to cut off the charging (by cutting AC feeding the the 6 watt motor) if the battery reaches
    voltage around 13.5 and start charging if voltage reaches down to 11.5. In this scenario the relay will be switching
    off and on and AC line.

    I beleive the circuit must be somehow connected to the battery so sense the voltage. My battery pack is
    Motolite 12v 200AH connected in serries.

    Thank you.


  3. Hi sir

    how much cuerrent wolud be at the output of the transistor?

    if i want to charged and to have the protection with the transitor but i want to charge it at 3A
    what should i do if a made a 13.8 V 3A power supply ?

    please tell me what resitance should i use?

    what i want is a way to leave the batery conected but it shunts when the voltage reaches the 13.8 V


  4. hi sir , canu help me out to made one battery dc to dc charger. my input voltage is 48v dc but frequency is little high. with this i want to charge lithium iron phosphate battery but i want to charge with high current approx 50A.


  5. Dear Sir,

    Please clarify the following

    1. Can I connect SMPS power supply at 18 volts 8 Amp as in put to this circuit.
    2.What is the maximum current that we can use to charge with this circuit.
    3.Can we use 40N03 MOSFET in place of 2N6292. If yes what modification is required? please give me the details

    With regards


    • Dear Raveesh,
      you can use 18V/8amps, if you are using a 7815 for the base drive of the BJT
      the current can be upgraded to any level by using an equivalently rated BJT.
      mosfet can be used but it will require at least gate voltage 6V higher than the full charge level of the battery, so that a correct voltage is able to reach the source until the battery is fully you may have apply 18V at the gate to get 14V at the source. This will require some practical verification.

  6. One more clarification

    If I use MOSFET, the battery to be charged is to be connected between source and ground or +ve supply and Drain. If it is between +ve supply and Drain, whether resistance is required between Source and ground. Please clarify

    With regards

    • If you want to follow the above concept, then it should be between source and ground, and make sure to use current input not higher than 2/10th of the battery Ah rating, if the battery is lead acid type.

  7. Dear Sir,

    The actual fact is, I have spare ATX PSU using TL494 and LM339 in which 12 volts output is rated at 8Amps and I have modified it to give variable out put up to 24 volts with the help of a pot & higher voltage output capacitors. The over voltage and short circuit protections are intact. I intend to use this as charger for my 70AH car battery. I have adjusted it for 14.2 volts and successfully tested it with 10 watt LED blocks(four in parallel) which draws more than 4 amps. The problem is, when I connect this to my car battery, the PSU senses short circuit and switches it off immediately. I would like to charge the battery at 14.2 volts/5 Amps at least. If I put 5 ohm 10 watt wire wound resister it will get too hot and Current and voltage will drop in the out put. How to overcome this problem with out drop in current and voltage. If you have any idea please help me.

    With regards


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