A 12V lead-acid battery will float charge at 13.8VDC. There are a couple of issues with connecting it directly across a 13.8VDC power supply. If the power supply gets shut off or looses primary power the battery may discharge back through the input of the power supply which can potentially damage the supply. A series diode is often used to prevent this back-flow of current. A silicon diode however drops about 0.7V in the forward direction so the power supply voltage needs to be increased to 14.5V in order to compensate for this drop. Schottky diodes and FETs will have a lower forward voltage drop and may be a better choice.
The other issue is that if you connect a fully discharged battery across a 13.8V source, the battery can draw a considerable amount of current until it obtains some charge. Automatic battery chargers contain a current limiter in order to prevent this. The heavy current is more likely to damage smaller batteries like a 7AH than a large capacity battery and of course doing it on a regular basis is more severe than occasionally.
Here's a link to a safe solution: http://www.westmountainradio.com/product_info.php?products_id=pg40s