My question is: Do the clock batteries on motherboards wear out quickly if you don't have them plugged in? I had both machines sitting around without any power for a while, the first one probably a month or so, the second one perhaps a year..
I managed to replace the PSU on the second one and now the fan comes on but I still don't get a signal from the monitor.. I'm thinking I should buy a new clock battery for it. Plus they're cheap so it won't hurt to try. But is this kind of thing common?
Clock (CMOS) batteries have a life of about 5 years regardless of whether or not the motherboard is plugged in. This assumes that when the motherboard is not plugged in, it is packaged correctly to prevent the battery from shorting out.
If you have reason to beleive the battery is dead due to age or shorting out, by all means replace it to eliminate this factor from the diagnosis. Good luck.
The drain on those batteries is going to be so low anyway that it doesn't matter whether the machine is plugged in.
It sounds like those machines are both fairly old, so I wouldn't be surprised if the battery went dead anyway. Replacing it is a good idea. 5 years is a good rule of thumb, but I've seen batteries last longer than that, and also much shorter -- on occasion they'll go dead in just a couple of years if you have bad luck.