One more thing....
I was looking at the P112K which includes the 624 PS. The specs don't list that as not allowing the battery to deep discharge. Do ayone know if it does or not? I did see Elk has a switch that could be wired in that has that feature.
Any recommendations for a secondary PS? Altronix?
Depends on what you're looking to do with it.
The smaller supplies don't offer any loadshed on AC loss/low battery, so $10 buys a LBC from Elk. The supply you purchase would depend on what your install has and what you intend on adding or using the supply for, or what you really want to do.
If you're running a general supply and want to supervise it, a 212S is a nice feature. If you're just running some minor things, I wouldn't justify the expense to drive a strobe and maybe a piezo or two, however if you size the supply and battery appropriately, loadshedding on the panel can be done. The big thing is when loadshedding burg devices (not audible/visual or fire) that your supply has more standby than your panel, otherwise very bad things happen as the supply cuts out with the panel still running, especially bus devices.