I have been working in the power industry for 15 years as an operations engineer. Power stations require DC (battery) power as a secondary / back up power source should the AC (mains / system power) supply develop a fault resulting in loss of primary power hence all power stations have battery banks to supply critical motors & control systems to safely shutdown the plant & then address the fault.
The point: the battery banks are considered a major hazard in the industry, classified as hazardous areas with restricted access requiring atmosphere monitoring. Batteries release hydrogen when charging, for that reason the batteries are housed in a separate enclosure with forced ventilation (ventilation fans).
Granted we are talking a much larger scale, but the batteries used are batteries you will find in lorries or similar just lots of them. The size of the enclosure & ventilation flow is all relative, as the enclosure is large & fans capable of displacing vast amounts of air, so a small battery in a small enclosure with no ventilation presents a similar sort of hazard.
The fans have redundancy built in, meaning 2 fans (1 duty, 1 standby) further demonstrating the significance of the hazard batteries present.
Often the vent fan running signal is fed back to the control system & alerts the control room staff of failure, this also cuts out the battery charger. Ie the batteries will not charge if no vent fan running.
Charging batteries in a cupboard with no ventilation in your home is far from ideal, hydrogen is the most volatile of gases (explosive range is 5%-75% volume in air) meaning any significant release of hydrogen is almost certainly going to become an explosive mix of fuel/ air.
Charging a large volume of batteries in a home coupled with the number of ignition sources in a house is asking for trouble, a hydrogen explosion is quite something, do a quick web search on hydrogen explosions, it may sway your choice of charging options.
A saying in our industry is 'routine wrong' it relates to things we know & do that are wrong but haven't had an incident, just because nothing has happened in the past it doesn't guarantee it won't happen in the future, luck is often the reason incidents have not occurred or on the flip side of the coin bad luck & poor judgement is the reason incidents occur.
Whichever nearly all incidents are avoidable, so why take the chance?
My advice is take heed of the comments leaving the battery in the car, if like the gentlemen in Wisconsin the ambient temperature isn't conducive to charging a battery follow the advice on the other comments.
Most importantly charge in well ventilated area.