AUTOMATIC 12V LEAD ACID BATTERY CHARGER

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

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.