Automatic 12V Lead Acid Battery Charger

07:34 2 Comments A+ a-

Schematic Circuit

This is a half of part of my final project circuit. I found this circuit at yesterday.  I hope this circuit can help me  completing my final project "Development of Flyback Converter for Battery Charging Application".

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.

  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.

Table Of Component:


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


Write comments
Huang Lissa
14 November 2012 at 01:41 delete

The acid should be diluted with an excess of water to give a 5 to 10-folddilution. The liquid should then be neutralized by mixing with an equal volumeof agricultural lime, or other alkaline material containing lead-acid batterycalcium carbonate, orsodium hydroxide. Use ofhydrated lime has the advantage of lower cost per unit of neutralizing capacitywhen compared to sodium hydroxide.

7 May 2013 at 17:46 delete

Heу ѵery interеsting blog!

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