# 12 Volt Car Battery Charger Circuit Schematic

## Circuit Description

Unlike many units, this battery charger continuously charges at
maximum current, tapering off only near full battery voltage. In this
unit, the full load current of the supply transformer/rectifier section
was 4.4A. It tapers off to 4A at 13.5V, 3A at 14.0V, 2A at 14.5V and 0A
at 15.0V.

## Circuit operation:

Transistor Q1, diodes D1-D3 and resistor R1 form a simple constant
current source. R1 effectively sets the current through Q1 – the voltage
across this resistor plus Q1’s emitter-base voltage is equal to the
voltage across D1-D3. Assuming 0.7V across each diode and across Q1’s
base-emitter junction, the current through R1 is approximately 1.4/0.34 =
4.1A. IC ensures that Q1 (and thus the constant current source) is
turned on.

When the battery has fully charged, the current through IC drops to a
very low value and so Q1 turns off (since there is no longer any
base-emitter current). R2 limits the current through IC. It allows
enough current to flow through the regulator so that Q1 is fully on for
battery voltages up to about 13.5V. Decreasing the value of R2
effectively increases the final battery voltage by raising the current
cutoff point. Conversely, a diode in series with one of the battery

## Parts:

• R1 = 0.32R
• R2 = 8.2R
• C1 = 10,000uF – 63V
• D1 = 1N4004
• D2 = 1N4004
• D3 = 1N4004
• Q1 = MJ1504
• IC = 7815 REG
• BR1 = 1N4004x4
• B1 = 12 Volt Battery

## Notes:

Charger’s input voltages are 20 volt AC
R1 and R2 are high wattage resistor like 2W, 3W, 5W and could be above.