12v Battery no Vent port

Yep... VW has used low voltage batteries inside the cabin of vehicles for near a 100 years without issue. There's no proven incident of thermal runaway with a 12v inside a vehicle that led to fumes in the vehicle that killed anyone unless there was some other type of fire or collision damage.

To further address your worries: 1) Amount of charge the Prius sends to the 12volt is so pathetically low it takes 12 hours of driving to recharge the 12volt whereas with all non-hybrid cars it takes less than an hour. 2) Because there's a robust battery venting system for the high voltage battery if it smokes, the 12volt, which sits next to the exterior vent of that system means there's already plenty of venting without the vent tube, 3) The Prius venting system for both batteries is so robust that starting in the early years of Gen3 Prius they eliminated that direct battery vent tubes from the hybrid battery pack and instead used the battery cooling system as the only needed venting.

 

What is the exact model of battery you bought?

It is difficult to make a recommendation with no information.

 

There's no proven incident of thermal runaway with a 12v inside a vehicle that led to fumes in the vehicle that killed anyone unless there was some other type of fire or collision damage.

Obviously, I don't know the true cause of the battery issue, but you never heard of Latifa and Maksmilla Lincoln? How much charge do you think the car was putting into the battery while they were driving down the highway?

Car Battery Fumes Blamed in Freak Roadside Deaths of Mom and 3-Year-Old Daughter: Report | Inside Edition

The very end of the article states:

"Despite the large numbers of lead acid batteries in use worldwide, no other fatalities of this type have been reported. Similar batteries, under experimental conditions, have been shown to produce hydrogen sulfide gas," the statement read.

"The battery was not the original battery for the vehicle, [or] the correct battery and is believed to have malfunctioned," the statement said.

 

Are their cabins as well sealed as Toyota's? My 2025 Camry with the battery in the trunk is vented because Toyota thought it necessary.
The OP likely got the wrong battery. All Prius batteries I have seen prior to Ge 4 were vented. The Gen 4 batteries were relocated under the hood, outside the cabin,

 

That specific case officially stated they couldn't rule it out as a contributing factor. But they also couldn't confirm it was the source. As in the battery was glitchy but not a total failure case where significant venting could occur.