# Lead-Acid Battery Charger Circuit

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This is the schematic diagram of Lead-Acid battery charger circuit. The circuit built around an IC UC3906 which is especially designed to give the appropriate charging voltage and current determined by the temperature and state of charge of the battery. A pass element carries the high current charge current within control of the charge controller.

Three indicators display the charge state. The charge controller drives the overcharge mode indicator directly. The electrical power indicator as well as the float state indicator are driven by voltage comparators. This keeps dissipation within the charge controller to a lowest so as not to influence the internal temperature sensor or have an effect on the output voltage programming divider.

The charger needs 18 to 22 volts DC input. This voltage could possibly be found from an 18volt notebook computer adapter. Optionally, populate the rectifier and the filter parts and supply 16 to 20 volts AC in the bridge configuration or 32 to 40 volts AC in a fullwave configuration.

Lead-Acid Battery Charger Parts List

 C1 = 47uF/35V C2 = .1uF/50V C3 = 47uF/35V C4 = .01uF/50V C5 = 3300uF/35V C6 = 3300uF/35V (>1A models only) C7 = .1uF 50V C8 = .1uF 50V D1 = Amber LED D2 = Green LED D3 = Red LED D4,D5,D6,D7,D8 = 1N5404 3A Rectifier U1 = LM2903 comparator U2 = UC3906 battery charger controller R1 = 22 ohm ¼ w 5% R2 = 0.13 ohm 1w 1% (2A), 0.25 ohm 1w 1% (1A), 0.5 ohm 1w 1% (0.5A) R3 = 33K ohm ¼ w 5% R4 = 1K ohm ¼ w 5% R5 = 2400 ohm ¼ w 5% R6 = 732K ohm ¼ w 1% R7 = 205K ohm ¼ w 1% R8 = 140 ohm ¼ w 1% R9 = 4.7K ohm ¼ w 5% R10 = 4.7K ohm ¼ w 5% R11 = 4.7K ohm ¼ w 5% R12 = 17.4K ohm ¼ w 1% R13 = 44.2K ohm ¼ w 1% Q1 = TIP125 NPN Transistor

Circuit Specifications:
Input voltage – one of the following:

• DC input voltage – 18—20V
• AC input voltage (Bridge) 16—20V
• AC input voltage (FW-CT) 32—40V

Threshold voltage – 10.2 V
Float -> bulk charge – 12.2 V
Bulk -> overcharge – 14.0 V
Overcharge -> float – 14.7 V
Float voltage – 13.9 V
Overcharge transition current – 0.2 A (.1A)
Bulk charge current – 2 A (1 A)

Download the full tutorial for lead-acid battery charger circuit project: