simple battery backup switch mosfet
Just as a idea because I don't know exactly what is your need (voltage in, voltage out, power level and so on), you can use a battery charger able to supply also load connected to the battery (e.g. Ansmann model ALCT 12-3) and a switching DC-DC post regulator (see attached circuit block schema). When the main power is down the battery will supply the DC-DC, if you rate in a right way the battery size with the output current I think that any small drop that you have on the battery side will be controlled by the DC-DC postregulator. About the DC-DC there is TRACO module that can be used for such purpose (e.g. 12 V in - 12 V out such as the TEN 20-2412WI 20 W module).
I've also attached the charger characteristics.
If you don't like to use such type of battery charger there is another way, use the main power supply and a power MOSFET with a voltage monitor (e.g. from Linear Technology), then from the main supply derive also a battery charger for the battery backup. When the main power go down the voltage monitor will turn on the MOSFET and use the battery as power supply for the output DC-DC. In this last way the backup battery will be attached to the output regulator in a fast way trought a sized MOSFET (of course voltage monitor and driver circuit will be feed by the battery itself).
Anyway I'm also searching such type of solution bcz I've a application where I can use main power line, solar cell and I must have also a battery backup such this you need. I'm investigatin some solution, also I've posted some schema on previous post but I've no reply about it, if you like take a look on this post:
Cheers
F.F. a.k.a. Powermos