I have a 12V 13A switch mode power supply that I'd like to connect to
a 12V lead acid automotive battery and use this combination to power
several amateur radio transceivers. The idea being that the battery
can supply peak current that exceeds the power supply rating and
provide emergency back up if power fails.

Is there anything I need to watch out for? Do I need a diode to
isolate the battery, or will the power supply rectifiers be enough? Is
13.8 V a good voltage to run the supply at? How critical is the
voltage setting?