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 TE Connectivity


  • Simple Auto-off 12V Battery Charger

  • Created: Aug 18, 2015

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Summary

In the automotive industry, lead acid batteries are commonly used. These batteries provide power to the vehicles for it to start and have the lightings on. Automotive battery comes in different voltage and ampere-hour ratings to fit different kind of automotive system. Common voltage ratings of automotive batteries available to the market are 6V, 12V and 24V. These are usually rechargeable and are charged during the vehicles run. When uninstalled, a charger can be used to recharge an automotive battery.


The project above is a battery charger with automatic off feature. The design is to be used on batteries with 12V voltage rating and 2.5Ah capacity. The capacity rating usually fits on motorcycle. The circuit can output a voltage enough to charge a 12V battery. A fully discharged 12V lead acid battery measures about 11.9V and a fully charged 12V lead acid battery measures about 12.7V. When charging, its voltage can rise to 15 or 16 V. However, it is important to limit the maximum battery voltage to prevent damage. The ideal value for the maximum voltage of the battery is about 13.8V to 14.4V. Beyond this range can cause gassing that can damage the battery. In this design, 13.8V is chosen to be the maximum voltage of the battery when charging. The battery is allowed to reach a floating voltage of 13.8V, under room temperature, to compensate self-discharge. When the charge in the battery reaches above 13.8V, the zener diode (D8) reversely conducts providing gate voltage to the SCR through the resistor-capacitor network. The SCR connects the coil of the relay to the ground charging it. Then the relay switches to open state which disconnects the battery from the charger to cut-off charging. The 1-1721956-4 relay is used in the circuit. It is SPDT in form and has 10A, 12V rating.


Improper charging of motorcycle and other automotive batteries results to a reduced calendar life of the battery. Care and correct procedures in maintenance of the batteries are needed in order to make its life span longer.

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