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

Schematic of the 12V automatic battery charger

Parts

Part
Total Qty.
Description
Substitutions
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)
Q11BT136 TRIAC
Q21BRX49 SCR
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

Notes

  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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Comments

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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
kc
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.)

Chris
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
jay
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
sushil
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 off..so plz if possible contact my email add..
Solartech
Automatic 12V Lead Acid Battery Charger
Sunday, December 29, 2013 12:38:57 AM
can this circuit diagram suitable for ups charger?
hafiz g
Automatic 12V Lead Acid Battery Charger
Wednesday, October 23, 2013 12:08:11 PM
asalamualaikum bro i have one prob on this circuit please tell me what is the substitute of BRX49 SCR ,,,,this ic is not available on my area so please guide me what can i use for its substitute.???
soundar
Automatic 12V Lead Acid Battery Charger
Monday, May 20, 2013 5:17:23 AM
hi..... how to add 2 or 3 batterys to this same circuit........???
faz
Automatic 12V Lead Acid Battery Charger
Monday, April 01, 2013 6:36:41 AM
i can't found the TRIAC BT136 but can i use TRIAC BT139 instead of TRIAC BT136? and can i use BC548 or BC558 instead of BRX49 SCR? and can i use IN4004 diode instead of P600 Diode? please help me. i need to complete my project urgent.
anonymsamaduSAloussa
Automatic 12V Lead Acid Battery Charger
Monday, April 01, 2013 5:24:52 AM
i assembled a 12v charger only using 1no center tap 12-0-12 transformer 4amp and 8nos of power diode 4nos for each side and charged 120AH BATT WITHIN 8HOURS. CHARGING CURRENT FOUND 10AMP. NO MORE COMPONENT USED. WHAT WILL BE THE DEMERITS ON IT .
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