battery backups with relays

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). 

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: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=255-1028-5-ND

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