How To Design Auto-Changeover Mechanism/System

All you need is a relay, the coil of which is powered by the mains supply and which will release upon failure of that supply. A pair of change over contacts is then used to provide the switching from one supply to the other. The common terminals connect to the common bus. The N/O contacts go to the main supply. The N/C contacts go to the standby supply. When mains power is available, the relay is energised and the N/O contacts are closed thus providing mains power to the bus. When mains is not available, the relay will be released, the N/C contacts will now provide backup power to the bus. Full isolation of both supplies is guaranteed with this arrangement.

This system will not automatically switch back to power if the battery becomes discharged whilst there is a power failure (not that it would do any good), but one would assume that you would use mains primarily, and size your battery bank to adequately cover any power outage period.

Personally, I would suggest a window comparator, to check each battery against the other. The highest charged battery would be switched into service. After the AC is reconnected, the comparator will switch to its normal state and both batteries will recharge and float at full charge.

Contact reputed manufacturers of changeovers/switchboards by specifying changeover delay time(in cycles or no delay)

Your problem is similar to battery bank management on sailboats (12-24VDC). Typically, of 2 battery banks, one is for house needs, and the other is for diesel starting. There is a lot of internet info available and a number of charger/regulator manufacturers. One of which is Bluesea. Google "Sailboat Battery Management" for ideas and see the link below for a variety of photos.

http://www.bing.com/images/search?q=Battery+Bank+Design&FORM=IRBPRS