Battery backup circuit help required!

[ali.mani]

hi. I am designing a project for vehicle tracking using sim548c module. the project is complete but the only thing remaining is that i want to develop a battery backup system for my circuit. i am using a 12v dc adapter to model a 12v car battery for the power supply. this 12v is used to power the sim548 module, a microcontroller and some other ICs. now i want a switching circuit which switches from the power supply to the external battery when the supply voltage falls below a certain voltage level, let us say 10v (taking into account a 12v car battery discharge). i was thinking of using a relay but could not find a 10v relay. kindly help me out!
(it is not required to charge the battery and currents upto 3A are required for the sim module)

 

[tpetar]

You can use the same relay for 12V, it will work on 10V in most cases. You can see manufacturer datasheet about that particular relay which you plan to use, what is minimal voltage for coil.

 

[iop95]

May be used a parallel connection between internal power supply and battery (eventualy small resistor for current limiting). In this way power supply keep charged battery, and feed power to circuit. If battery is externaly charged or have a separate charging circuit, may use a diode (2 or 3 in series) to isolate battery from internal power supply (if needed / required). When power supply drop below 11.3V (10.6 or 9.9V), automaticaly battery will became source for circuit.