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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Matthew Schembri Automatic 12V Lead Acid Battery Charger Wednesday, October 28, 2009 3:48:17 PM

Hi, can the circuit be run from a 12V dc source or does it have to be AC ?

swisskid Automatic 12V Lead Acid Battery Charger Saturday, October 17, 2009 5:39:13 PM

i av found the substitute for brx49 i actually used BT169 its difference is that BRX49 can accommodate a max voltage of 200v while BT169 can accommodate up to 400v but they both do pretty much the same work. i av used it in my circuit and it worked. the only problem that i av is that i need a modification for 24v charging system.

user Automatic 12V Lead Acid Battery Charger Monday, October 12, 2009 4:10:48 PM

You can try 2N6565 for BRX49. See http://www.classiccmp.org/rtellason/transdata/2n3001.pdf Notice the authorship of the schematic is not known. Please stop posting comments hoping someone will write you back. If you don't want this circuit to drain your battery if it's connected and unplugged, buy one more D1, and stick it between Q1/R1 and the battery, with the cathode connected to the battery side, and the anode connected to the component side. If you drew it on the schematic its symbol would be thus: Q1 & R1 ->|- to 12V battery.

Alan Mann Sr Automatic 12V Lead Acid Battery Charger Saturday, September 19, 2009 9:17:56 AM

I have put together this charger with these parts, DigiKey part # R1,R3= OD33IJE-ND R2 =100OHM 1/2W 3329P-101LF-ND pot R4,R5,R7,R8= 82W-2-ND R6= 100XBK-ND R9= OD102JE-ND C1= P13477-ND D1= SB550-E3/54 GICT-ND D2= 1N4004 DICT-ND D3= 1N4734A DICT-ND D4= LED Q1= 568-3651-5-ND Q2= EC103M-ND T1= TRIADMAGNETICS P#= F219X 115/12VAC 4A Now out put of transformer is 13.65VAC and out of D1 is 8,95VDC. On Q1 pin 1, 8.95VDC Pin 2, 1.05 which is the 12V + bat out put. Pin 3 Gate, 8.96. Q2 is Pin1, 8.3-V Pin 2 .41V Pin 3 -.2V, which is the -12V bat out put. So what have I done wrong? Any Help would be appreciated. I am trying to make a charger for my robot which has 4 6V SLA batteries 2 in series and then in parallel for 12V.

anonymous Automatic 12V Lead Acid Battery Charger Tuesday, September 08, 2009 12:32:38 PM

There is a problem, if you leave the charger at an unattended location and the power goes out, it will discharge the battery.
(Editor's notes: Yep.)

Sachin Automatic 12V Lead Acid Battery Charger Tuesday, September 08, 2009 11:36:32 AM

can i know the actual pin configurations of the triac and the scr used in the circuit?plsss.... PLEASE GIVE ME THE SUBTITUDE OF BRX49 scr i am from india and it is not aviable plz give me the another subtitude for it. I used tyn604 but LED is glowing Countinusly. please send me details and please send pcb layout. thank you.

Garouda Automatic 12V Lead Acid Battery Charger Thursday, August 20, 2009 6:03:16 AM

Why do you use a triac in a DC circuit? What about replacing D1 by a rectifier and Q1 by a thyristor?

cedric Automatic 12V Lead Acid Battery Charger Tuesday, August 11, 2009 9:45:34 AM

what other equivalent can one use to replace the thyristors if not found?

chandrashekar g Automatic 12V Lead Acid Battery Charger Wednesday, July 29, 2009 10:46:50 AM

sir i need brx49 scr but in local market not avilable in this number scr plase help me this scr eqlenet numer send my mail your sinci chandrashekar

Rachael Automatic 12V Lead Acid Battery Charger Monday, July 20, 2009 7:10:20 AM

Ya. its nice. i request you to kindly explain for me how it works. Theory and procedures used. Use my email please. Thank you.
(Editor's notes: Search the forum for this explanation.)

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Matthew Schembri	Automatic 12V Lead Acid Battery Charger	Wednesday, October 28, 2009 3:48:17 PM
Hi, can the circuit be run from a 12V dc source or does it have to be AC ?
swisskid	Automatic 12V Lead Acid Battery Charger	Saturday, October 17, 2009 5:39:13 PM
i av found the substitute for brx49 i actually used BT169 its difference is that BRX49 can accommodate a max voltage of 200v while BT169 can accommodate up to 400v but they both do pretty much the same work. i av used it in my circuit and it worked. the only problem that i av is that i need a modification for 24v charging system.
user	Automatic 12V Lead Acid Battery Charger	Monday, October 12, 2009 4:10:48 PM
You can try 2N6565 for BRX49. See http://www.classiccmp.org/rtellason/transdata/2n3001.pdf Notice the authorship of the schematic is not known. Please stop posting comments hoping someone will write you back. If you don't want this circuit to drain your battery if it's connected and unplugged, buy one more D1, and stick it between Q1/R1 and the battery, with the cathode connected to the battery side, and the anode connected to the component side. If you drew it on the schematic its symbol would be thus: Q1 & R1 ->\|- to 12V battery.
Alan Mann Sr	Automatic 12V Lead Acid Battery Charger	Saturday, September 19, 2009 9:17:56 AM
I have put together this charger with these parts, DigiKey part # R1,R3= OD33IJE-ND R2 =100OHM 1/2W 3329P-101LF-ND pot R4,R5,R7,R8= 82W-2-ND R6= 100XBK-ND R9= OD102JE-ND C1= P13477-ND D1= SB550-E3/54 GICT-ND D2= 1N4004 DICT-ND D3= 1N4734A DICT-ND D4= LED Q1= 568-3651-5-ND Q2= EC103M-ND T1= TRIADMAGNETICS P#= F219X 115/12VAC 4A Now out put of transformer is 13.65VAC and out of D1 is 8,95VDC. On Q1 pin 1, 8.95VDC Pin 2, 1.05 which is the 12V + bat out put. Pin 3 Gate, 8.96. Q2 is Pin1, 8.3-V Pin 2 .41V Pin 3 -.2V, which is the -12V bat out put. So what have I done wrong? Any Help would be appreciated. I am trying to make a charger for my robot which has 4 6V SLA batteries 2 in series and then in parallel for 12V.
anonymous	Automatic 12V Lead Acid Battery Charger	Tuesday, September 08, 2009 12:32:38 PM
There is a problem, if you leave the charger at an unattended location and the power goes out, it will discharge the battery. (Editor's notes: Yep.)
Sachin	Automatic 12V Lead Acid Battery Charger	Tuesday, September 08, 2009 11:36:32 AM
can i know the actual pin configurations of the triac and the scr used in the circuit?plsss.... PLEASE GIVE ME THE SUBTITUDE OF BRX49 scr i am from india and it is not aviable plz give me the another subtitude for it. I used tyn604 but LED is glowing Countinusly. please send me details and please send pcb layout. thank you.
Garouda	Automatic 12V Lead Acid Battery Charger	Thursday, August 20, 2009 6:03:16 AM
Why do you use a triac in a DC circuit? What about replacing D1 by a rectifier and Q1 by a thyristor?
cedric	Automatic 12V Lead Acid Battery Charger	Tuesday, August 11, 2009 9:45:34 AM
what other equivalent can one use to replace the thyristors if not found?
chandrashekar g	Automatic 12V Lead Acid Battery Charger	Wednesday, July 29, 2009 10:46:50 AM
sir i need brx49 scr but in local market not avilable in this number scr plase help me this scr eqlenet numer send my mail your sinci chandrashekar
Rachael	Automatic 12V Lead Acid Battery Charger	Monday, July 20, 2009 7:10:20 AM
Ya. its nice. i request you to kindly explain for me how it works. Theory and procedures used. Use my email please. Thank you. (Editor's notes: Search the forum for this explanation.)