This is modified version of commonly available automatic car battery charger system. I obtain original layout of this charger from one of the commercially available Chinese car battery charger and modified some of the sections of original schematic to improve the performance and stability of the system. This charger is mainly based on LM311 comparator and 2 × 12V 4A transformer.

Automatic car battery charger finished prototype
Automatic car battery charger finished prototype

All the components of this car battery charger are commonly available in the market. Ammeter in this system must be withstand for minimum of 8A current. Low cost ammeter can also be used with suitable shunt. For the relay it is highly advisable to use some high quality unit. During our prototyping we mainly use 12V Omron and Panasonic realys with this system. At the test stages we observe that some of the cheap relays may start to oscillate heavily with this system.

Under normal conditions TIP142 and 7805 regulator in this system may not need any heatsinks.

Final view of automatic car battery charger
Final view of automatic car battery charger

This unit is specifically design to charge 12V lead acid car batteries and it is not suitable to use with other battery types.

Project Related Downloads

Schematic diagram, PCB design files and wiring diagram of Automatic Car Battery Charger project.


LM311 - Single Voltage Comparator
TIP142 - Complementary Silicon Power Darlington Transistor
LM7805 - 5V Positive-Voltage Regulator