What about in cold climates? Can you leave wet cell lead-acid batteries outside when the temperature goes below freezing? Or below zero degrees F? What is the freezing point of the diluted sulfuric acid solution in a wet cell battery?
Many big battery manufacturers produce extended temperature range batteries for this sort of installation, however, it's more common to provide a bit of heat to keep them in their "happy" range.
Often, the bigger issue is adjusting the float voltage to stay in the manufacturers recommended range based on battery temperature.
Most decent systems will have at least one battery temperature probe and will have built in temperature compensation control.
Off the shelf UPS systems won't have this, so it's something to consider. While putting batteries outside may be required, looking at how to best vent is another. All batteries will eventually vent some hydrogen, even the so called "sealed" batteries. Most of the stuff you guys are talking about are actually Valve Regulated Lead Acid type (VRLA) and will have a vent on the cell(s) that will open at a certain pressure level.
Keeping the float charge voltage set correctly, temperature compensation, low voltage disconnects, and sizing everything properly will drastically reduce the chances of venting enough hydrogen to be an issue, however it should always be considered a risk.
I'm running -48vdc systems with 1800AH battery stacks. All of these systems are indoors. All of them have hydrogen sensors, ventilation systems and are temperature controlled.
Usually the way I design my systems, hydrogen venting in enough volume to trigger the sensors means I've either done something really stupid, or the system has failed in some way.
