OK. I've got some old APC equipment and I'd love to know how to do this. Write up?
I looked into using APC equipment but 1: it's VERY expensive to ship to Canada, and 2: I read stuff about them slightly overcharging batteries, some even say they do on purpose so you need to buy new ones more often, but I suppose that's left for a whole other debate and lastly 3: I did not specifically try this with an APC but with an Ultra, and the issue is that there is some sort of pre programmed shut off timer. Though I have heard of this done with higher end APCs so I suppose it can be done.
To avoid those potential issues I got an inverter-charger instead, it's a device designed to act as a UPS with bigger batteries. So instead of doing something that is completely against the UPSes warranty or even design, this device is made for this.
This is my current setup:
The cover is normally left on as to hide the terminals and keep the batteries relatively clean from dust and other debris. There are holes for the cables and to act as ventilation.
That black box is the inverter-charger, it's a Tripp Lite 750w unit but they make bigger ones too. In fact now that I look at their site I kind of regret not going with the 48v model.
http://www.tripplite.com/en/products/product-series.cfm?txtSeriesID=818
I plan to improve my setup though as it's kinda hackish atm. I'll be building a battery shelf so they can be stacked on, with clear plastic ventilated covers on both sides.
That said if you want to experiment with old APC stuff, simply ensure you know the voltage it requires (probably 24v) and get deep cycle batteries like these and simply hook them up in series. I bought these at Canadian Tire and also bought the cables there. The only thing I'm not sure is how you'd permanently hook it up to the APC. I used crocodile clips when I was testing, but it's not something I'd ever recommend for a permanent setup. The higher end rack mount APCs use 48v I believe. If you still have the batteries then it's most likely as simple as checking how many batteries and how they are configured as each battery will be 12 volts. (should say on the label). I've seen some APCS that have "battery packs" which is basically 4 batteries stuck together with two sided tape.
Beware of hydrogen. You want to ensure whatever charging device you use wont overcharge them.