The battery charger part of the circuit is also completely automatic so when the battery will become full charge the charging will be automatically stopped which will be indicated by the green LED used in the circuit. The output current is 1A due to which the circuit can be used for wide variety of devices which require current under 1A.
The circuit is using a 7.2AH SLA battery but you can also use a 10AH or 12AH battery for long back. You can also use 20AH or 25AH SLA battery but it will take a longer charging time as compare to the 7.2, 10 and 12AH batteries. For example a 20AH 75% discharge battery will take around 20 hours for full charge.
The circuit can be easily modified for higher ampere output and for charging higher ampere batteries faster by using a high ampere transformer with suitable diodes in the place of all 1N4007, and using a transistor with LM7812 to increase the output current.
The battery charger part of the circuit requires some adjustments at the first time. For adjusting connect an adjustable power supply and set its voltage to 14.4V. Now remove the battery and the 1A transformer from the battery backup circuit and connect the power supply in the place of battery and adjust the 10K variable resistor until the green LED lights up. After these adjustments connect your lead acid battery and the 1A transformer with the battery backup circuit and remove the adjustable power supply, when you connect battery with the circuit it will go in charging mode. Now check the battery with multimeter while it is charging and see if the green LED lights up when the battery voltage reached to 14.4 while charging. If not then again adjust the 10K variable resistor so the green LED will light up when the battery voltage will be 14.4 while charging.
The figure below shows a project of simple automatic 12V battery backup power supply circuit. The circuit will automatically shift the load to the battery when the mains supply is not present, and when mains supply is back the load will automatically shift to the mains supply and the battery will go in charging mode.
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