battery backups with relays

Hello all,
      I am doing a project which needs two seperate 12v batteries to power up two units one at a time. one battery(working) gives power to two units simultaneously and when the voltage level of that battery is reduced around to 9V the system needs to be switched to the other battery.How can i do this? Can i use relays to do that? If so please give me specifications of the relay i need to find.
        Thanks


-------------
asithwenura

What kind of loads are you working with?  How much current will they draw?  What kind of batteries will you be using?  How will you be charging them?

The short answer is yes, you can do it.  The long answer explains why it is probably a bad idea to go this route because the relay coil will only add to the load on the battery and cause the battery to drain that much quicker.

One way to make it happen is to use the secondary battery to power a small battery voltage circuit.  Idealy, the circuit would run at 5vdc off of a regulator that has a small quiescent current (not a typicaly 7805).  The circuit would include a comparator and a voltage divider.  The voltage divider would monitor the voltage on the primary battery divided by 4.  This value (the primary battery voltage / 4) would go to one side of the comparator.  The other input on the comparator could be a 5vdc 10K pot.  You could then set the pot to a voltage that is equal to your desired battery switching point divided by 4. 

The theory behind the voltage divider is to keep the voltage going in to the comparator from rising above the 5vdc power supply.  With the divider dividing the battery voltage by 4 you would be able to charge the battery all the way to 20vdc without worring about damaging the circuit.  Say you want a 9vdc setpoint, you would then set the 10k pot to 9/4 = 2.25vdc.

The output of the comparator would then drive a transistor that would drive a relay, preferably a reed relay or some other low current relay (possibly even a solid state relay). 

-------------
Kevin Pierson

Thanks Kevin for your quick reply.
I am using a 12/7 Lead acid battery and plans to recharge by 230vAC commercial power. Each battery needs to power up a 5V RF transmitter (433MHz). The overall circuit may draws 1 A from the battery . Can you please explain me why you have mentioned that 5Vdc should not be obtained by 7805. then how can a regulated 5vdc can be obtained. can you please further mention any known relay that suits for my need which can be bought in the market. Since i don't have a clear idea about the specifications need to be checked in purchasing a relay it's a big help if u can say something.
           thanks and hope to hear from you soon.

-------------
asithwenura

7805s are not very efficient and will draw "excessive" current.  You can get more efficient regulators, like the LM2931.  This is important in this situation because I'm assuming you want maximum battery life (based on your requirements).  If max battery life isn't important you could go with a 7805 and save a few pennys.

If 1A is all you need then you should be able to find a 5vdc 2A relay (twice your requird current).

Here is a relay that only needs 28.3mA at 5vdc: https://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=255-1028-5-ND

For your current requirements this might be a good voltage regulator: https://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=576-1106-ND

-------------
Kevin Pierson

Kevin Thanks for your help. i'll give a try and tell you the results soon.

Thanks

-------------
asithwenura

If you want to get 5V from a 12V supply, a linear regulator is going to have to dissipate a lot of power and heat; figuring that 1 amp is going to be 12V-5V = 7V x 1A - 7W.   When you're only going to be using 5W of that for your circuit, it may be worth going to a switching DC-DC converter.

It'll be more components to deal with but it'd probably be worth the effort considering the amount of voltage drop you're looking for.

This is something you might check into- 12V DC-DC step down converter
Page 10 shows a typical application circuit to get a 5V 1.5A output, so you could use that for your purposes.

Just came across this interesting item, a prepackaged 7805 pin compatible switching regulator-
10W step down switching regulator

Costs more at $15, but it's all done for you, and looks like a tidy little package.
It has a screw adjustable output, and can output 1.25 to 10V @ 1A, using any input from 3 to 30V, with an efficiency of about 87% converting 12V to 5V, which isn't too shabby.

dear kevin & dualsport,
        Thanks for ur great ideas to fulfil my requirement. keep your work continue...

-------------
asithwenura