But are there any problems keeping it enclosed for gasses or other reasons?
Unless something is wrong with it, there should be nothing released from it. Hot gases are released in a fire, but at that point it doesn't really matter.
Will the charging itself generate a lot of heat so that the box actually need some cooling (like a fan drawing in colder air) despite the outside air being below freezing? Any thoughts on this?
If it heats up that much during charging, then it's being charged beyond the real capabilities of the cells (they are too high a resistance, so a lot of charging current is being converted into waste heat). (Temperature affects the resistance, too).
If that happens I'd recommend using a lower-amp charger.
Some heat will always be generated as there is always some resistance in the cells, but it should be minimal with good cells in good condition. If you can feel heat coming off a pack when charging it at room temperature, it's either not a very good pack or it's being charged at too high a rate for it's actual capabilities, or both.
You can calculate the pack resistance with some guesstimates for interconnect resistance gathered from the various spotwelding threads and multiply/divide by the number of series and parallel connections (one on each end of each cell, plus whatever series and parallel interconnects there are), and if you can find out the actual cell model that was supposed to be used in your pack, you can look up it's datasheet for it's resistance, and calculate the total cell resistance based on the number of parallel and series cells.
Even the now-ancient (decade-and-a-half old) EIG cell packs I use don't have a noticeable temperature change during charge or discharge. The total pack resistance estimated by the Cycle Analyst is less than 50mohms, so a 12A charge rate at 57v max (is more like 50-52v while its at the 12A rate) makes 12A x 0.05ohms x 52v = 31.2watts, generated within a volume of around a foot x a foot x 3/4 of a foot. (don't recall the exact measurements). 12A is a bit more than 0.25C for these packs; IIRC 0.5C is the max rate recommended for them when new (and since they're very old that should be lowered to accomodate their changing properties).
this page
Use the watts to heat calculator to know the power you need to heat a particular substance.
www.omnicalculator.com
gives me a guesstimate of about 7F temperature change for 35lbs of mass if I use aluminum (a large part of the mass of the cells, AFAIK) for the material, assuming a constant 31w (it won't be, it will decline) over half an hour (the maximum time it typically will be near max current during a charge, dropping off after that so the temperature change rate is even lower). Since it's not insulated to keep heat in (or out) the pack won't heat up as much as that, as it will radiate and conduct away some of that heat. (I don't know how to calculate that and didn't find an easy calculator for it).
They're built into the SB Cruiser trike so in the "cold" part of winter for a couple of weeks I sometimes use an incandescent lamp under the trike to keep the seatbox and contents warmer than it would otherwise drop to just so they will perform better on my morning commute, because there isn't enough heat generated internally to keep them at a good riding temperature to minimize voltage sag under load.
Consider that the cells are at most risk of fire due to overcharging or overcurrent; that is to say, a battery made with high quality cells kept indoors at rest is at almost no risk of fire. Not zero risk, I'm not saying that, but certainly much lower risk.
It's a UPP pack, so there's probably not much chance of that.
(or of high quality construction, or BMS)
