This is a project of a automatic battery charging timer circuit. The circuit is able to charge many types of batteries from 5 to 12 volt. The circuit shown here is used for charging a 12V lead acid battery.
The circuit is divided in two stages the first stage is a battery voltage monitor circuit which will monitor


the battery and start the timer on a preset voltage. The preset voltage can be selected with the help of the 10K variable resistor. The second stage is a timer circuit that is built around a 555 timer IC.

For charging low voltage batteries for example 5 or 6 volt change the input DC power supply to 9V DC or use a 9V transformer in the place of 12V, and change the zener diode to 2.4 or 2.5 volts. The transformer output ampere will be according to the battery type. Slow charging is always good for battery life, therefore it is better to charge a 10AH battery with a 1 or 2 ampere transformer in 5 or 10 hours.

Here is a simple calculation that will show you how to know the charging time of a battery with a transformer. The calculation is easy, just divide the AH of the battery with the output ampere of the transformer like this:

Battery AH / Transformer ampere = Required hours to complete charge.
10 / 2 = 5

For example if you are charging a complete discharge 12V 10AH battery and your transformer output is 2 ampere. Then first adjust the 555 timer circuit with 5M variable resistor to activate the output relay and LED till 5 hours.

But the circuit is using a battery monitor circuit which will not let the battery completely discharge once you have adjusted the 10K variable resistor. For example if you have adjusted the 10K variable resistor to start the charging on 9.5 volt then you do not have to charge it for 5 hours. Only adjust the timer circuit to charge the battery for 4 hours. Once these settings are done the charger is ready to automatically charge the battery whenever its voltage reached to 9.5 volt. For charging the batteries of 6V use 5 or 6V relay, a 12V relay will not work on voltages lower then 9.

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Automatic Battery Charging Timer Circuit Diagram