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battery backup circuit schematic

Battery Backup Circuit

This battery backup circuit can be added to surveillance systems like alarms and others to power the circuit during mains failure. The battery backup will immediately take up the load without any delay.

The circuit is simple to construct. Regulator IC 7812 gives 12 volts regulated DC for powering the circuit as well as to charge the rechargeable battery. LED indicates the power on status. When the mains power is available, diode D1 forward biases and passes current into the battery through R2. Value of R2 is selected to give 90 mA current (12/100 = 0.1A) for slow charging.

When the mains power fails, D1 reverse biases and D2 forward biases and backup the circuit. The same circuit can be used in circuits having 6 volt 7.5Ah battery. For 12 volt battery, use 7812 regulator IC and 14 Volt input.

Battery Backup Circuit Schematic

battery backup circuit schematic

battery backup circuit schematic

26 Comments

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  • David Cyr

    You will charge the batteries, but you will also be feeding 8.4V (9 – 0.6) to the circuit; not 7.4V. Forget this circuit, it just won’t work as advertised, regardless of which components you use.

  • srinu

    Sir… I used two mobile phone li-ion battery’s with voltage 7.4.. Bt this circut cant recharge di battrys…. Plz give me reply… I used 7809 ic instead of 7812….

  • srinu

    Sir… I used two mobile phone li-ion battery’s with voltage 7.4.. Bt thus circut cant recharge di battrys…. Plz give me reply

  • David Cyr

    Don’t bother trying to use this circuit. It won’t charge your battery! You need much more than 24 Volts to charge a 24 Volt battery. I am sure the author who has now disappeared never used this circuit, because it won’t work and I am sure he never actually tried it.

  • Afimilk

    hi,

    what changes do I have to do in order to adjust the circuit to one with input of 30VDC, 24VDC output, and 24VDC 7AH battery?

    THANKS!

  • David Cyr

    We have built a backup battery system as shown above but instead of the 7812, we power the circuitry and charge the backup battery directly from the aircraft alternator and main battery. Also, there is no need for R2 since the aircraft voltage regulator maintains the main battery voltage at close to 14 volts while running. Given the presence of diode D1, the backup battery sees approx. 13.3 volts, which as it turns out is not sufficient to charge the backup battery. Have you found your backup battery actually charges with this circuit?

    BTW, your assertion that R2 will charge the backup battery with 90 ma is not correct, as the only way 90 ma would flow through R2 is if the backup battery voltage were zero!

  • Melkske

    Can i have a load of 3A?

    Regards
    Geert

  • MarkSH

    Good circuit but I am interested to know why 1N4007 diodes are used when 1N4001 could be used and also have smaller dimensions?

  • Favour

    Pls how would my project switch to the battery from the main supply and back to the main suppy in case of power failure

  • Narendrapal singh

    Sir we require 9 volt 1 amp dc supply with 6 volt 4.5 ah rechargeable
    battery backup, when mains off output remain 9 volt without unintruped.
    We have to use this supply to operate weighing scale.

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