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

Parts

Part	Total Qty.	Description	Substitutions
R1, R3	2	330 Ohm 1/4W Resistor
R2	1	100 Ohm 1/4W Pot
R4, R5, R7, R8	4	82 Ohm 2W Resistor
R6	1	100 Ohm 1/4W Resistor
R9	1	1K 1/4W Resistor
C1	1	220uF 25V Electrolytic Capacitor
D1	1	P600 Diode	Any 50V 5A or greater rectifier diode
D2	1	1N4004 Diode	1N4002, 1N4007
D3	1	5.6V Zener Diode
D4	1	LED (Red, Green or Yellow)
Q1	1	BT136 TRIAC
Q2	1	BRX49 SCR
T1	1	12V 4A Transformer	See Notes
F1	1	3A Fuse
S1	1	SPST Switch, 120VAC 5A
MISC	1	Wire, Board, Heatsink For U1, Case, Binding Posts or Alligator Clips For Output, Fuse Holder

Notes

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.

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.

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.

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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cetot Automatic 12V Lead Acid Battery Charger Tuesday, August 10, 2010 12:55:39 PM

This project is interesting...but how to modified the schematic to become the Automatic variable battery charger 0-24VDC with the cutoff switch when the battery is FULL loaded? If better to add meter and display to indicated the charge voltage and current...Any idea to done this?

ali raza Automatic 12V Lead Acid Battery Charger Wednesday, July 14, 2010 9:04:50 PM

hi i have to install battery charger for my ups so if i have to increase current of charger i.e 4amps to 10 or 20amps what i have to do??

anonymous Automatic 12V Lead Acid Battery Charger Tuesday, June 29, 2010 8:38:47 AM

I am surely interested to build this project, however my great problem is how to find replacement for SCR- BRX 49. Do you have any suggestion? Can I add a relay switch to an inverter to switch to it during power brownouts?

namnp2007 Automatic 12V Lead Acid Battery Charger Tuesday, June 22, 2010 8:41:34 PM

Why we need R5 and R8 here? I thing it the same as R4 and R7.

rounak Automatic 12V Lead Acid Battery Charger Friday, June 04, 2010 3:31:52 PM

hi, can i use TYN612 instade of BRX49 SCR? because BRX49 SCR is unavailable only TYN612 available.please hurryup sir i m on line...

anonymous Automatic 12V Lead Acid Battery Charger Wednesday, June 02, 2010 4:13:38 AM

So this circuit can charge any sealed 12V battery? What about higher voltages? Would this design work to maybe charge electric scooter batteries?

anonymous Automatic 12V Lead Acid Battery Charger Monday, May 31, 2010 11:29:24 AM

To avoid battery discharging in event of mains faiIure, fit a relay whose (230v) coil is energised by the input supply, and switch the charger output + via the N/O contacts of the relay. clsed when power present, open when not. Simple, cheap.

Steve Quah Automatic 12V Lead Acid Battery Charger Sunday, May 16, 2010 7:07:11 PM

You can avoid discharging the battery when the power is off by adding a diode at the output +ve. By the way the circuit does not work with Full-wave rectification right?
(Editor's notes: Adding the diode would prevent this circuit from working.)

Ajin vishnu A.S Automatic 12V Lead Acid Battery Charger Tuesday, May 11, 2010 10:09:32 AM

This circuit is a good one. I can easy understand it.

JPL Automatic 12V Lead Acid Battery Charger Monday, May 10, 2010 9:05:57 AM

Would leaving the battery connected to the circuit while not plugged in be cured by putting a diode between the charging circuit and the battery on each terminal? If so, what should be used

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cetot	Automatic 12V Lead Acid Battery Charger	Tuesday, August 10, 2010 12:55:39 PM
This project is interesting...but how to modified the schematic to become the Automatic variable battery charger 0-24VDC with the cutoff switch when the battery is FULL loaded? If better to add meter and display to indicated the charge voltage and current...Any idea to done this?
ali raza	Automatic 12V Lead Acid Battery Charger	Wednesday, July 14, 2010 9:04:50 PM
hi i have to install battery charger for my ups so if i have to increase current of charger i.e 4amps to 10 or 20amps what i have to do??
anonymous	Automatic 12V Lead Acid Battery Charger	Tuesday, June 29, 2010 8:38:47 AM
I am surely interested to build this project, however my great problem is how to find replacement for SCR- BRX 49. Do you have any suggestion? Can I add a relay switch to an inverter to switch to it during power brownouts?
namnp2007	Automatic 12V Lead Acid Battery Charger	Tuesday, June 22, 2010 8:41:34 PM
Why we need R5 and R8 here? I thing it the same as R4 and R7.
rounak	Automatic 12V Lead Acid Battery Charger	Friday, June 04, 2010 3:31:52 PM
hi, can i use TYN612 instade of BRX49 SCR? because BRX49 SCR is unavailable only TYN612 available.please hurryup sir i m on line...
anonymous	Automatic 12V Lead Acid Battery Charger	Wednesday, June 02, 2010 4:13:38 AM
So this circuit can charge any sealed 12V battery? What about higher voltages? Would this design work to maybe charge electric scooter batteries?
anonymous	Automatic 12V Lead Acid Battery Charger	Monday, May 31, 2010 11:29:24 AM
To avoid battery discharging in event of mains faiIure, fit a relay whose (230v) coil is energised by the input supply, and switch the charger output + via the N/O contacts of the relay. clsed when power present, open when not. Simple, cheap.
Steve Quah	Automatic 12V Lead Acid Battery Charger	Sunday, May 16, 2010 7:07:11 PM
You can avoid discharging the battery when the power is off by adding a diode at the output +ve. By the way the circuit does not work with Full-wave rectification right? (Editor's notes: Adding the diode would prevent this circuit from working.)
Ajin vishnu A.S	Automatic 12V Lead Acid Battery Charger	Tuesday, May 11, 2010 10:09:32 AM
This circuit is a good one. I can easy understand it.
JPL	Automatic 12V Lead Acid Battery Charger	Monday, May 10, 2010 9:05:57 AM
Would leaving the battery connected to the circuit while not plugged in be cured by putting a diode between the charging circuit and the battery on each terminal? If so, what should be used