I know humidity is ‘bad’ for household AAA batteries, but what exactly would or does happen?
I’ve never heard that humidity is bad for batteries. Exposure to liquid water could be though (and high humidity can result in the condensation of water, so I guess it follows)
The cases of household cells are typically made from rolled steel, which can rust. The containers are designed to withstand a certain amount of internal pressure arising from the chemical processes inside - rust may weaken them sufficiently to permit them to rupture.
I haven’t seen a failed battery since the case was lead and the center post was carbon.
IIRC, those were actually lead-acid batteries.
The acid inside could eat through the lead cylinder. If you find an ancient flashlight with the battery compartment badly corroded, those old batteries were responsible.
I cannot imagine a modern battery giving a rat’s patooie about moisture.
p.s. - I saw a camera built in or shortly after 1985 with that corrosion. Have not seen anything more recent.
Actually the latest alkaline batteries seem to have had a resurgence of badly leaking examples. I’ve no idea why, some seem to think it’s a result of manufacturers somehow cheapening the materials or construction. Carbon-zinc batteries are still available but not much utility.