Automatic 12V Lead Acid Battery Charger

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This charger will charge any 12V lead acid battery including flooded, gel and AGM. It is fully automatic and will charge at a rate up to about 4A until the battery voltage reaches a preset point at which it will switch to a very low current float charge. If the battery voltage drops again the charger will begin charging until the voltage once again reaches the cut off point. In this way it can be left connected to a battery indefinitely to maintain full charge without causing damage. An LED indicates when the battery is fully charged.


Schematic of the 12V automatic battery charger


Total Qty.
R1, R32330 Ohm 1/4W Resistor
R21100 Ohm 1/4W Pot
R4, R5, R7, R8482 Ohm 2W Resistor
R61100 Ohm 1/4W Resistor
R911K 1/4W Resistor
C11220uF 25V Electrolytic Capacitor
D11P600 DiodeAny 50V 5A or greater rectifier diode
D211N4004 Diode1N4002, 1N4007
D315.6V Zener Diode
D41LED (Red, Green or Yellow)
T1112V 4A TransformerSee Notes
F113A Fuse
S11SPST Switch, 120VAC 5A
MISC1Wire, Board, Heatsink For U1, Case, Binding Posts or Alligator Clips For Output, Fuse Holder


  1. R2 will have to be adjusted to set the proper finish charge voltage. Flooded and gel batteries are generally charged to 13.8V. If you are cycling the battery (AGM or gel) then 14.5V to 14.9V is generally recommended by battery manufacturers. To set up the charger, set the pot to midway, turn on the charger and then connect a battery to it's output. Monitor the charge with a voltmeter until the battery reaches the proper end voltage and then adjust the pot until the LED glows steadily. The charger has now been set. To charge multiple battery types you can mount the pot on the front of the case and have each position marked for the appropriate voltage.
  2. Q1 will need a heatsink. If the circuit is mounted in a case then a small fan might be necessary and can generally be powered right off the output of D1.
  3. T1 is a transformer with a primary voltage appropriate to your location (120V, 220V, etc.) and a secondary around 12V. Using a higher voltage secondary (16V-18V) will allow you to charge 16V batteries sometimes used in racing applications.
  4. If the circuit is powered off, the battery should be disconnected from it's output otherwise the circuit will drain the battery slowly.

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Automatic 12V Lead Acid Battery Charger
Sunday, July 12, 2015 7:44:07 AM
the half wave rectifier will not give the required 13.8v charging voltage if it is feed by the 12v ac from T1.
Automatic 12V Lead Acid Battery Charger
Wednesday, April 15, 2015 8:35:59 PM
I built this circuit more than 20 years ago and I used two SCR and a bridge graetz. I can guarantee that near zero volts the Q1 was dying anyway and left again at the next half-wave, of course, if the circuit demanded it. Sometimes I used a Tr NPN instead of Q2, but I never used Zener diodes, resistors only calculated appropriately. A defect and a merit of the circuit: while the charge current is reduced, the components are heated and increases their conduction thereby reducing the V Gate (or VBE).
Automatic 12V Lead Acid Battery Charger
Thursday, February 19, 2015 5:29:06 PM
hello I would like to ma information about the R2 component ... need to know if it is 100 Ohms or 100k Ohms eh ?? do not think this variable resistance of 100 Ohms here in Brazil I can use a 100k in place of that of 100 Ohms or some other
Automatic 12V Lead Acid Battery Charger
Wednesday, January 28, 2015 8:11:56 AM
HI can this charger modified to give the battery charging level (more than one green led and one red during charging process).the charger can use the solar modules .If yes pls send me the circuit diagram and component list.Also the price of this kit. thanks
Automatic 12V Lead Acid Battery Charger
Sunday, November 23, 2014 7:55:00 AM
Thank you for the great Circuit. LED goes off at about every 10 seconds. Is it the normal state of operation or do I need to adjust Pot to glow it continuously. ?
Abid Ali
Automatic 12V Lead Acid Battery Charger
Saturday, April 12, 2014 2:20:05 AM
hi! if 18 AC volts Can we use this circuits And one battery charge from solar and other battery fit in running circuit, the solar battery can connect to the running circuit battery. can you help to design a circuit for this. thank x
Automatic 12V Lead Acid Battery Charger
Thursday, March 06, 2014 8:07:17 PM
Hello. Just thinking what is the output power of this circuit? And how long will it take to fully charge the 12V battery? :) Thank you.

(Editor's notes: As I'm sure the notes say, the output is dependent on the size of the transformer up to 5A. And the time it takes to charge the battery depends on the size of the battery.)

Automatic 12V Lead Acid Battery Charger (how can I increase the Ams to 12-14amps?)
Thursday, January 30, 2014 10:57:00 AM
Hi, Thank you for sharing this knowledge. Can you please help me with an information? I need a 12 volt battery charger with 12-14 amps capacity. From the schematic above, what are the components I need to change to achieve this? and what exact specs of the components? Many Thanks. Chris
Automatic 12V Lead Acid Battery Charger
Tuesday, January 28, 2014 6:19:49 AM
i have a question. what if there is no available BRX49, is there any option that this circuit work? or what would i do of this problem?? please help me, i need an answer.. you are part of our project.. please reply asap. Godbless
Automatic 24V Lead Acid Battery Charger 150 mAh batt. each
Sunday, January 05, 2014 3:22:18 AM
thanks for your above chart it is clear but I really need 24 volts to charge 2 batteries at same time so plz can you just replace the required parts only on Description table or whole image itself. Actually I got 220 ac to 24 volt ac transformer i want to use this transformer for charging by converting into DC line..i n past I did lot but always some thing blows plz if possible contact my email add..
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