12v starter battery AGM or fully sealed

First, I have only used the factory replacement battery purchased directly from Toyota dealership -- and -- of course, it has the proper vent port to exhaust outside.

No expert here, but I understand the basics of this. A conventional "flooded" battery has the potential of off-gassing. So does an AGM, but very much less likely and very much less off-gassing -- at least in a perfect world. Of course, this is not a perfect world.

If pressure builds, an AGM WILL vent and if you don't have the tube to direct it outside, it will all be floating around inside the Prius.

So, in my opinion I would NOT USE any battery without properly venting it to the outside.

I have read more than a few posts/articles online where folks have just installed a non-vented battery and not worried about build-up of gas in the car -- which I admit many people claim is unlikely -- but that does not mean it won't happen.

So, again, myself. I would not risk it. Just depends on how much of a risk-taker you are.

In a nod to Mendel, here's a Canadian link that explains some of this:

Flooded versus AGM Batteries?Flooded versus AGM Batteries?

Clint Eastwood movies are too violent for me -- but my father loved them and I was lucky (?) to get to sit through most of them with him -- one moment stands out and seems to fit this situation:

 

i agree, went with the oem, but haven't read any horror stories about unvented.

 

Because Prius 12v don't get used the same way as a regular car uses a 12v, there's an opportunity to rip people off with a "specialized battery" that's different from a standard 12v automotive battery.

So a former PriusChat battery expert in @Britprius gave it a hard look and realized that mobility batteries for wheelchairs have way more amp/hour capacity than what they sell for Prius and also have a more extreme safety design standard because they are used in homes and hospitals. Disabled people literally sit on top of these 12volts all day long, even when they're being charged. What's more, the prices for these batteries are much more competitive and don't have the BS claim of a specialized Prius battery to jack the price up.

I put one of these 55amp/hour AGM non-vented 12v in my 07 Prius in January of 2013 and it wasn't till this past November 2021 that is was bad enough to replace. Of course watch out for Priuschatters who will get fanatical about these 12volts not having a tiny little emergency vent tube. We've been arguing about the value of that concern on here for over a decade now. Here's the 12v I recommend: 12V 55Ah Rechargeable SLA AGM Battery for Wind, Solar, and Off Grid Power | eBay