100 miles per week to keep the 12V battery from failing

What's your parking situation, would you be able to reliably hook up a charger?

Another option, in particular if a charger is impractical, is some sort of cut-off switch, to isolate the 12 volt battery when the car's off. It is a bit of a hassle, and may lose some settings, say radio presets, for one.

You could check what the "background" load is on the battery, basically you put an ammeter in series between the negative battery post and the car body, with the car fully "asleep". Around 20 milliamps is what you should be seeing, if there's grossly more, figure out why and deal with it if possible.

 

Not really horsehockey; frequent short hops will kill a regular car battery too. The real problem is that new ECU dependent cars draws more electricity, with key-less entry and push-button starter constantly scanning for your FOB. OEM's has also been putting smaller batteries into their cars at the same time. Your dealership could be mishandling those new batteries too. They are heavy and a lazy technician or stock clerk can easily slam them around; chipping away at the longevity of them. New batteries are sent out only half charged; so if they install a new battery without placing a full charge on them and your doing short-hops; not running the car long enough to recover the startup and down time between use - your slowly chipping away at the battery. It's like plugging in your cell phone for two minutes and expecting it to run for a week with that short recharge interval.

Solutions:
Place the car on an RV trickle charger when your not using it or at least overnight.
And/or install a BM-2 battery monitor. This is a Bluetooth device, with a phone app that will give your battery status when your in range.
https://www.ebay.com/itm/800183124112?_skw=bm2+battery+monitor&itmmeta=01KWYNJF5E7AJ15EQMPQESWAJ6&hash=itemba4ea18090:g:hEQAAeSw4OBpbJwn&itmprp=enc%3AAQALAAAA0LB2JXbe%2Bh2pdWBqbR2c2DvDrkIgYaInyZ6mJW0KcFTc7VuyTjpbaE%2FMrtSWVuC0AIfjeMfdlh3i1Uc1HfojpX5ilZs7iVQ8n2o0wbcDxlP6MwvBk1iBFu5FeTHvrjcF8XhtYiLiUwtEl%2BjOBLIBHvZebQ930gPfhUvQeQXe2ZIgPHu3lHJX1KgwHiLY9atpLSr12Px2%2BmWGNY9g9eNo9otFfAuo6dCtkrI1bnRDzAv6wGaXDFACyiJaGlLq0uQvTnq3TMeLgp%2FERw42eWQgojo%3D%7Ctkp%3ABlBMUPTyydXnZw

 

there are a few threads here regarding this problem that many are experiencing. read through them, there might be a helpful software update iirc, but it may not be the final solution.
unfortunately, toyota dropped the engineering ball on this and is ignoring it and its customers.

 

With 4 batteries in 15 months that tact seems to be mostly flapping the sails? It could well be excessive background drain. Again I would check that, and consider a charger. We're driving 3k kms a year, at the most, and the 12 volt battery was installed in September of 2015. It pretty much lives on a charger.

 

When did Toyota add a “Low battery” indicator for anything other than the key fob battery?

 

I see what your trying to say here, but you running over a nail is NOT the dealerships or OEM's fault. That's like saying the dealership is responsible, if you get into an accident driving the new car home - they DON'T owe you a new car.
Toyota has sold 56.5K 2025 Prius; if this was a manufacturer's defect - the NTSB would be looking into it. You should probably start filing lemon law paperwork - since this is their forth shot at fixing it. Hopefully your state has car lemon laws on the books.
Have they even tried looking for a root cause; ie. run REAL diagnostics to determine if software is up to date and/or there's no parasitic current draws? If they are just replacing the batteries w/o looking for the root cause, are you selling it before the warranty runs out? What's the plan here? I believe dealership flat rate is north of $150/hr and tracking down a parasitic drain is at least a few hours; it's going to hurt if that happens on your dime - Just saying.
Battery is covered under your 3 year or 36K mile bumper to bumper warranty - only the power train is covered for 5 years or 60K miles. The battery is NOT part of the power train warranty.

Good Luck....

 

It pops up when people are in accessory mode and the 12V battery draws down.