Speaking as one who knows some theory but hasn't
had extensive experience living offgrid with
a battery bank to maintain:
If you're actually charging / discharging the batteries they'll
put out quite a bit of heat. Good insulation and merely ordinary
ventilation should let them keep themselves adequately warm.
Don't sweat freezing. While low temperatures reduce the output
voltage, it's really hard to freeze a lead-acid battery even if
it's just sitting there rather than putting out heat from charge/
discharge. (Have you ever had one crack in your car - even at
minus four farenheit?) The acid is a pretty good antifreeze and
there's plenty in the water unless you've discharged your batteries
below the safe limit for decent battery lifetime (in which case
you'll be replacing them soon anyhow. B-) ) Also: Even if they
ARE nearly flat they're a considerable thermal mass. So they
won't freeze quickly even on a minus-degree night.
You DO want to lift the bottom layer off the floor -
especially a concrete floor - by putting it in a rack
or putting something insulating under it. Because the
voltage varies with temperature, sitting a battery on a
concrete floor (or other strong thermal mass) can result
in a temperature difference between the top and bottom of
the battery. This leads to uneven charging, with the warmer
part discharging itself to try to overcharge the colder
part - with bad effects on lifetime and capacity. (To a
lesser extent selective blocking of air circulation might
cause a similar issue.) Keeping the battery off the floor
(like on open racks where air can circulate evenly around
the cells) mitigates this.
(This also lets you put an acid-proof drip catcher under
the whole rack so a leak won't put holes in the floor.)
For the same reason: If you've got a battery with cells
paralleled you'll want the paralleled cells to be at about
the same temperature - so put them at the same height in
the racks.
