Actually Astron power supplies (and a few other manufacturers as well) have issues when you attempt to float batteries on their output. There are modifications available - check out Repeater-Builder. In addition, the output voltage will likely need to be adjusted down, unless you enjoy spending money on battery replacement. If your intention is to float-charge batteries, do yourself a favor and buy a supply that is designed to do so. If the batteries will live inside, buy batteries that have a vent with fittings to allow you to vent any products of outgassing to the outside world.
If there is sufficient ripple present on the output to be noticeable, you have a very cheap power supply. The RS-12 seen here is specified as <5mV peak-to-peak under the worst conditions (full load, low line voltage). Adding batteries won't help - you will still see the ripple, since the power supply will almost certainly be at a higher potential than the batteries. I have used any number of power supplies (although I typically buy decent quality stuff) and have never seen an issue where ripple has affected anything I was working on.
I have an Astron RS-50A on the bench for high-current work along with a VLS-20M for variable output voltage up to 30V. I've also got a lab-grade voltage/current controlled HP supply for delicate stuff. I've been reasonably happy with Astron's switch-mode supplies as well, and have used a few Samlex switchers with no ill effects.
To the OP: You have a decent, but small supply. At 12A max output (at "ICS" - 5 minutes on, 5 minutes off) you can run 2 55-watt rotators, one strobe power supply of reasonable size, or damn near a whole LED lightbar (don't turn the halogen takedowns/alleys on). The fuse you're seeing is the input - 115V @ 3A. If you overload the supply, the worst thing that will happen is it will shut down - the output voltage will fall to zero. I wouldn't leave the supply sitting like this for hours, but you won't hurt anything. And, from your post, it looks like you basically pulled this thing out of a junk box, so even if it does blow up, what's the harm? Do yourself one favor... find a good voltmeter and check output voltage at rest and under load - make sure something funky isn't going on. It may be in the junk pile for a reason, and you don't want to toast something expensive. The output voltage should remain a fairly steady 13.xVDC until you get close to the max output of the supply, where it may fall off a bit.
When you start playing with electronics, you will inevitably smoke something sooner or later. I've done it, we all have. Don't do something stupid (shorting batteries/hooking them up backwards, touching exposed AC lines, etc.) but don't be too scared to learn, either.
