Lead Acid Battery Charger Schematic Diagram
Except for use as a normal Batter Charger, this circuit is perfect to 'constant-charge' a 12-Volt Lead-Acid Battery, like the one in your flight box, and keep it in optimum charged condition.
This circuit is not recommended for GEL-TYPE batteries since it draws to much current. The above circuit is a precision voltage source, and contains a temperature sensor with a negative temperature coëficient.
Meaning, whenever the surrounding or battery temperature increases the voltage will automatically decrease. Temperature coëficient for this circuit is -8mV per °Celcius. A normal transistor (Q1) is used as a temperature sensor.
This Battery Charger is centered around the LM350 integrated, 3-amp, adjustable stabilizer IC. Output voltage can be adjusted with P1 between 13.5 and 14.5 volt. T2 was added to prevent battery discharge via R1 if no power present.
P1 can adjust the output voltage between 13.5 and 14.5 volts. R4's value can be adjusted to accommodate a bit larger or smaller window. D1 is a large power-diode, 100V PRV @ 3 amp. Bigger is best but I don't recommend going smaller.Parts List:
R1 = 120 Ohm C1 = 100uF/63V Q1 = NTE374/BD140
R2 = 82 Ohm C2 = 10uF/63V Q2 = NTE123AP/BC547
R3 = 10K D1 = 1N5401/NTE5801 U1 = LM350 (On large coolrib!)
R4 = 33K D2 = LED (Red, 5mm)
R5 = 22K
P1 = 2K2
Source : http://www.sentex.ca/~mec1995/