Chager circuit for SMF batteries.

admin June 21, 2008 21 Comments


Here is a simple charger circuit for charging SMF(sealed maintenance free ) batteries.The charger circuit being curernt and voltage regulated,can be safely used to charge SMF batteries of 1.2 AH rating.This circuit was actually designed in response to a request from a reader.With slight modifications you can use it for charging any sort of SMF batteries.

The first part of the circuit is the power supply made of transformer T1 and diodes D1&D2.The capacitor C1 filters the rectifier output .The next part is the section comprising of IC1 and Q1 which provides the necessary volatge and current regulation.The diode D3 prevents ther reverse flow of charge from the battery .

Circuit diagram with parts list.



  • Assemble the circuit on a good quality PCB or common board.
  • An ammeter can be conncted in series with the battery to monitor the charging current.
  • Do not short circuit the charging terminals of the circuit.The circuit doesnot have short circuit protection.
  • R4 is the component that determines the charging current.Here it is set to be 120 mA. R4= (0.6 V/Charging current).By selecting the proper value of R4 according to equation, you can charge batteries that require different charging currents.
  • pavan
    May 21, 2014

    hi,one need 14.1 v to charge 12 v battery,so input to lm 317 should be 3+14.1=17.1 v .there fore transformer suppose to have 18 v as output

    • Seetharaman
      May 22, 2014

      14.1 volt DC. A transformer with 14 volt RMS secondary will produce after rectifying and smoothing a DC of RMS secondary X root 2(1.414) = 19.8 volts hence it is sufficient. if you use a 18 volt transformer it will produce 25.5 volts DC. The IC has to drop more than 10 volts hence it has to dissipate >10 watts. It will get heated up.

  • March 24, 2014

    hi want to charge 12v 7 ah battary what changes to required to this circuit

    • Seetharaman
      March 26, 2014

      transformer rating to be raised to 2amps. all diodes should be 1N5401. R4 should be reduced to 0.47 ohms 2 watts fuse before battery should be 2amps. ensure a good heat sink for the regulator ic.

  • Albin George
    December 8, 2013

    Is there is any substitute for the 2n4123 transistor?

    • Seetharaman
      December 9, 2013

      You can use any general purpose NPN transistor similat to BC548. 2N2221, 2n2222 etc

    August 13, 2012

    very useful simple circuit thanq

  • Mufazzel Sarangpur Wala.
    December 22, 2011

    We require chargers for 12V 9 Ah batteries on a regular basis, our initial order is 500 pcs , please contact me on zenithagro at hotmail dot com or zaindus at hotmail dot com.


    Mufazzel Sarangpur Wala.
    INDORE (M.P.) -INDIA – 452002.
    Phone : 0731-2542821.
    Mobile : 09303227028 / 09981457475.

    • jojo
      December 23, 2011

      Hello Mufazzel,

      We dont make any chargers for commercial purpose here. This circuit is for educational purpose. You may contact industrial vendors for purchasing needs.

  • seetharaman
    November 19, 2011

    Hi RAM you require a regulated 14volt DC supply with the current limiting fixed at 5Amps max. Use LM338K in place of LM317. Use 5 Amp 15volt transformer with D1 to D4 as 6A10 rectifiers. Reduce R4 to 0.12 ohms 5 watts. Raise the fuse rating to 5Amps.

  • ram
    November 19, 2011

    hi i want to charge a SMF battery of 12volts and 26Ah plz send me the charger circuit schematic

  • Jitendra mattolia
    January 7, 2011

    I have used ic 555 for chrging SMF 12V-7.5Ah. It works exellently. May i share this ciruit with u?

  • Oliyide Rafiu
    June 2, 2010

    Thanks for the info, but i will kindly need a charger to charge a 12V 100A SMF battery

  • April 18, 2010

    Thanks for sharing.
    How do we modify this circuit to charge one or multiple units of SMF 5-volt motorcycle batteries. I will appreciate your advice about this project

  • Kamran S
    August 17, 2009

    Hi. I need a slow Ni-MH battery charger design with minimu, parts count. Can you help? Here is the Spec.:

    Battery pack voltage: 43.2V, 1,000mAH
    Available DC voltage: 45-46VDC
    Charging speed: Slow, at 30-50ma
    Trickle charging: Yes

    Kamran S

  • Johnn
    July 25, 2009

    What is the secondary voltage from the T1 transformer?

    If the battery has a 1.2 AH rating should you use a 1 amp fuse in the circuit!?!

  • athar
    February 21, 2009

    plz mension the input&output

    • admin
      March 28, 2009


      • August 16, 2012

        kindly write the specification of the components please it is a humble request.

      • Anjan
        July 6, 2014

        Hi Admin Please Write the list of the specific component with their value..

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