contrary to what others say, this is not a trickle charger...and while it puts out 13.8 vdc nominal it is most likely not well regulated and is NOT a battery charger. Given it's age and lack of use, you may have bad caps in it as well.
The more current the little gem is asked for, likely the lower the voltage it will provide. It may or may not have shunting or shutdown...difficult to say with out a model number. That can damage sensitive gear.
It was made to use with automotive equipment - lights and some of the sniffer/switches may fail with use of this supply.. all depends on how lucky you feel. You would be better served to use the battery with a real trickle charger in place..
this is not marine quality and may not play well with your gear.