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12V 100Ah Lead Acid Battery Charger Circuit

12V 100Ah Lead Acid Battery Charger Circuit

The proposed 12V 100ah lead acid battery charger circuit was designed by one of the dedicated members of this blog Mr. Ranjan, let's learn more regarding the circuit functioning of the charger and how it could be used as a trickle charger circuit also.

Update: For an Auto-Cut off Lead Acid Battery Charger, please refer to this Article

The Circuit Idea

My self Ranjan from Jamshedpur, Jharkhand. Recently while googling I came to know about your blog, and become a regular reader of your blog. I learned a lot of things from your blog. For my personal use I would like to make a battery charger.

I have a 80 AH tubular batery and a 10 Amps 9-0-9 volts transformer. So I can get 10 amps 18-0 volts if I use the two 9volts leads of transformer.(Transfomer is actually obtained from an old 800VA UPS).

I have constructed a circuit diagram based on your blog. Please have a look on it and suggest me.  Please note that,.

1) I am belonging to very rural area hence there is a huge power fluctuation it varies from 50V ~ 250V. Also note that I will draw very less amount of current from the battery( Generally using LED lights during power cuts) approx 15 - 20 Watt.

2) 10amps transformer i think safely charges 80AH Tubular Battery

3) All diodes used for the circuit are 6A4 dides.

4) two 78h12a used as parallel to get 5+5 = 10 amps output. Although I think Battery must not draw full 10 amps. as it will be in charged condition in day to day use so internal resistance of battery will be high and will draw lesser current.

5) A switch S1 is used thinking that for normal charge it will be kept in off state. and after fully charging the battery it switched to on state to maintain a trickle charge with lower voltage. NOW question is that is this safe for the battery to kept in charge unattended for long time.

Please reply me with your valuable suggestions.

12v, 100ah battery charger circuit diagram designed by Mr. Ranjan

simple lead acid battery charger circuit

Solving the Circuit Request

Dear Ranjan,

To me your 12V 100ah lead acid battery charger circuit design looks perfect and should work as expected. Still for guaranteed confirmation it would be advisable to check the voltage and current practically before connecting it with the battery.

Yes, the shown switch can be used in the trickle charge mode and in this mode the battery can be kept permanently connected without attending, however this should be done only after the battery has been fully charged upto around 14.3V.

Please note that the four series diodes attached with the GND terminals of the ICs could be 1N4007 diodes, while the remaining diodes should be rated well over 10amps, this could be implemented by connecting two 6A4 diodes in parallel at each of the shown positions.

Also, it is strongly recommended to put both the ICs over a single large common heatsink for better and uniform thermal sharing and dissipation.

Caution: The shown circuit does not include a full charge cut-off  circuit, therefore the maximum charging voltage should be preferably restricted between 13.8 to 14V. This will ensure that the battery is never able to reach the extreme full charge threshold, and thus remain safe from over charge conditions.

However this would also mean that the lead acid battery would be able to attain only around 75% charge level, nevertheless keeping the battery undercharged will ensure longer life for the battery and allow more charge/discharge cycles.


About the Author

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!

43 thoughts on “12V 100Ah Lead Acid Battery Charger Circuit”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. The author mention on this article two 78h12a used as parallel to get 5+5 = 10 amps output. that mean if I parallel more 7812 I may get 5A more from each regulator ic?
    coz i have a 8v-0-8v 40A transformer ( rewind a old MOT transformer) and Iam looking for a circuit to make a 13.8V battery charger from it without losing the transformers current value.


    • yes that’s correct you can add any number of ICs in parallel and get the added value for the max current, just make sure to mount all of them over a common heatsink, and in that case you can eliminate the output diodes….

    • Hari, I already have a few Arduino based inverter circuits in this website, please find them through the search box…

  3. Pls I am finding it difficult in connecting my charging circuit to the inverter bcos when the inverter starts working its drows current from the charging circuit rather than to drop from the battery… Which courses the rectifier diodes to burn out …. Please help me solve it

    • use a relay to isolate the (+) line of the battery….join the coil of the relay with the charger supply….connect the pole of the relay with the battery….N/C with the inverter positive input…and N/O with the charger supply (+)

  4. Good day Swagatam I am a beginner and want to build a 12v 100ah battery charger. I have a transformer from a 650v UPS 1N5400 diodes and a 1000uf 35v Capacitors. But the Transformers and diode gets too hot when connected to the battery. Pls can you suggest a simpler setup? Have been trying to figure out the circuit above. Thanks

    • Good day Charles,
      it's because your transformer is trying to stuff as much as possible current into the battery which is making the devices hot and this might also damage your battery.

      you will have t include a current controller stage in the middle.

      you an use a simple transistorized current controller stage for limiting the charging current to the battery to 10 amps, using the following circuit


      you will simply need to replace the LED points with your battery points.

      however the bridge rectifier diodes cannot be 1N5400, these will need to be at least 20 amp rated rectifier diodes.

  5. Hi, you can easily do it with a 36V, 20 amp solar panel…..just plug it in, it will charge your battery within 6 hours (assuming the sunshine is optimal)

  6. hi, can you help to design solar battery charger for 30vdc-100ah battery bank (6vdcx5-100ah) based on mosfet and opamp

  7. Good day Sir, my mind still confused about this circuit,, we all know that 14.4v are needed to charge a 12v battery so why 7812?? i know that this is a 12v regulator? am i missing something?

    • Joseph, the 4 diodes connected with the center pin of the 7812 elevates the output by 0.6 x 4 = 2.4V, that's equal to 12 + 2.4 = 14.4V

  8. good day mr. swagatam..i was intersted to built cgarger same as above diagram using two 78h12A and using 18v transformer.this diagram make me easy specialy begginer like me..but i want the output would be 30 to 50 amp.this will be very helpful to my shop..please give me diagram for this same as mr. ranjan diagram..and also correct material..tnx

  9. My 600va ups transformer secondary volt only 9v
    can i make bettery charger for charging 100ah bettery.problem is beacause its give only 9v.

    • Hi Anish, just connect the meter in series with the positive wire of the panel….

      (+) of the meter goes to the panel positive while the negative of the panel goes to the load positive….

  10. how to measure secondary winding and primary winding in my ups transformer one side blue and red wire and other side yellow,blue,brown and black. i plug ac volt in blue and red wire but this is shorting and blow my board fuse.
    its intex ups 600va.

    • get an inductance meter and measure the values of the winding, the one which gives the highest value would probably be the primary mains…

  11. Dear Swagatam, please I need you to help me with a 12v self charge inverter of 2 and 5kva with 220v output. I mean without any input

  12. "it can be replaced with a relay
    and an associated circuit for obtaining
    an automatic response"

    Quoting you from above sir, Is there such circuit in your database that I can add to this circuit for such?

    Another question Sir….

    Please is there a reason you don't make SCR battery charger circuits?

    I can't seem to find any on your blog sir.

    I came across some chassis bolt mount scrs and I was hoping you might have made chargers with them.

    If so, please do you have such circuit?

    • Adding an auto-relay in the above circuit could make it much complex, you can make the following circuit for getting all the required automatic actions in one:

      For implementing trickle charging actions just connect two 1N5402 diodes in series from the main +12V to the circuit positive rail with a 1K series resistor

      The lower relay is not required it can be removed.

      SCR concepts are not as efficient as relay/transistor chargers, therefore I did not find them useful enough to be included

  13. What kind of switch would be employed here sir?
    Could it be replaced with a relay and pots for setting trickle charge/batt full cut-off and low voltage start-up thresholds?


    • the switch can be an ordinary spdt switch, it can be replaced with a relay and an associated circuit for obtaining an automatic response.

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