12V UPS power supply and battery
You would only have backfeed current if the battery was at a higher voltage than the power supply leads. This is highly improbably has most batteries will not hold a charge higher than they are spec'd.

Personally I have a deep cycle SGM battery in parallel with a 12V PSU (floats at 13.8). and I have never had any problems. 

The only major concern that I know of is overheating of the battery, causing it to vent noxious fumes, but as long as you arent shorting the battery or drawing insane amounts of current you should be fine. I still wouldn't recommend a homebrew solution like this for anywhere in your home, its an ok garage UPS though.

However if you want to use this anywhere where people live, I would definitely put some sort of diode in, http://www.digikey.com/product-detail/en/APT75DQ120BG/APT75DQ120BG-ND/1494832 would be sufficient, it can handle the voltage drop from 13.8 to 12. It has at least a 10* reverse stand off (this is my ultra safe rule of thumb). and it can handle double your amperage load.

Purely because this is dangerous I would officially recommend buying a professional UPC.