Charger Circuit for 6V or 12V Car Battery

Charger Circuit for 6V or 12V Car Battery


The circuit has been designed to construct a practical mechanism that can easily and automatically charge batteries rated at 6 V or 12 V used in automobiles.

Terminology BC557 – PNP general purpose transistors used for amplification and switching due to its low current and low voltageBC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selectionsBD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximum2N3055 – a complementary Silicon Epitaxial-Base planar NPN transistor mounted in Jedec TO-3 metal case for use as power transistor Circuit Explanation

One of the essential factors in the operation of this circuit is the utilization of transformer denoted as T1 which has excellent resistance and insulation for short circuits making it a good quality. It is responsible for transferring electric energy or alternating current from one circuit to another via conductors that are coupled inductively. A transformer is composed of two or more coils of conducting element, like a wire, wrapped around a core where a magnetic field is produced by an alternating current in one coli. This will induce the same current in the other coils.

The transistor Q1 is conducting through voltage divider R1 and R2 while trimmer TR1 and resistor R4 are functioning as regulated current source. The Darlington pair of power transistors Q5 and Q6 is being driven by a current through R9. A Darlington pair is designed in such a way that the collectors of both transistors are attached together and the emitter of the first transistor is directly coupled to the base of the second or output transistor. The base current of the output transistor is equal to the emitter current of the input transistor. A car battery charger is normally around 6 V to 8 V while the current is around 1.2 A being handled by the trimmer TR1. As the battery is charged slowly, the voltage is gradually increasing until it reaches 7 V, which would cause D1 to conduct. The increase in voltage results to the decrease of voltage across R3 which makes Q1 to conduct. The process will continue until the circuit reaches 6 A. the transistor Q4 will be driven by the voltage drop via R10. Since the transistor Q5 is grounded, the excess current across it would remain constant for modification.

When the charging applied reaches 14.4 V, the circuit parallel to the battery will be triggered, which consist of R6, D2, D6, and D8. The indication of complete charging of the battery will show in LED D8 as it conducts simultaneously upon charge completion. This will also lead to Q2 turning ON due to voltage drop across R6. Some of the currents at the base of Q5 will be grounded by Q3 as it conducts. The current across the base of Q5 will be very minimal when the voltage reaches around 15 V, which will stop the battery for charging. To protect the battery from short circuit or high voltage, diodes D5 and D6 are used while D6 is used for misplaced error from battery poles. In case an error occurs, D9 will turn ON for indication. Diode D2 which possess 6.8 V can be shorted by closing switch S2 for charging of 6V battery.

The charging current of 1.2 A should be adjusted by the trimmer TR1 with the use of a 6 V battery. An ampere meter with 10 A rating will be connected in cascade with the battery. In the absence of a 6 V battery, a charger can be used that will pass through the ampere meter to achieve the adjustment of TR1 to 1.2 A. When a 12 V battery is used, switch S2 will be open during the adjustment. The values of diodes D2 and D3 should be precise to protect the battery from overcharge. An acceptable value of 100 mV can be considered as the voltage deviation. If TR1 is not enough and current adjustments are causing difficulties, the value of R4 can be altered until the desired charging current becomes 1.2 A. two parallel resistors formed by R10 is placed with an appropriate distance from the printed circuit board, Q5 and Q6 since its temperature increases during operation. Because of this fact, Q5 and Q6, together with bridge B1, should be placed on a heatsink with the isolation of a suitable silicon mica capacitor. Since the current in the circuit is large, the PCB and bridge should be connected with thick and short cables, as well as using a wide enough PCB. The whole circuit should be enclosed in a metal casing with dimension that has enough ventilation.

Part List

R1-11=1K ohm 0.5W 5%
R2=22K ohm 0.5W 5%
R3-5-8=10K ohm 0.5W 5%
R4=2.2K ohm 0.5W 5%
R6=100 ohm 0.5W 5%
R7=100K ohm 0.5W 5%
R9=470 ohm 0.5W 5%
R10=0.08 ohm 10W  5W
B1=Bridge Rectifier 25A/40V
D1-2=6.8V 0.4W Zener
D3=4.7V 0.4W Zener
D5=18V 0.4W Zener
D8=LED 5mm Yellow
D9=LED 5mm Red
Q5=BD139   [On Heatsink]
Q6=2N3055 [On Heatsink]
TR1=4.7K Trimmer Pot.
C1=4700uF 40V
C2=1uF 25V
T1=230Vac//15V 10A Transf.
F1=Fuse 1A Slo Blo [5X20mm]
S1=2X2 Switch 10A per contact
S2=1X2 step mini switch
J1…4=Flat Pin Connector
J5=6pin Connector 2.54mm
A=0-10A Ampere meter
Batt=12V or 6V Battery

Important Note

It is very important to note that dealing with batteries will always bring danger of explosion, so proper handling and care is a must.


The theory behind the construction of this circuit may be applied not only for charging batteries of cars but also motorcycles, personal watercraft such as Jet Ski, boat, and yacht, batteries of snowmobile, motor scooter, riding mower, utility vehicle, RV and marine application, wheelchair and mobility scooter, Ups/telecom, broadband and cable TV, solar deep cycle, four wheeler, quad bike, and ATV.

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