Note:
Also make sure to use a relay with some pretty heavy duty contacts. The capacitors on the system side of the relay (the COM terminal) need to be large enough to supply the current requirements to the system during the relay change-over time. This could be as much as 50 or 100 msec for some heavy duty relays. There is the disadvantage that the capacitors demand a lot of current flow in the relay contacts when the system is turned on for the very first time.

I would suggest that you may want to consider raising your SMPS voltage to be above or the same as the battery voltage and then switch these through the relay and put a common LDO on the system side of the relay. If you raised both the battery and the SMPS voltages another volt or so then you could eliminate the relay entirely and use two power diodes to summ the battery and SMPS outputs together to the input of the LDO regulator.

Michael Karas