2014 Prius new 12v batt won’t hold charge.

Dead

 

Don't know without some testing. Could be a loose or corroded main 12 V cable too.

Mainly, charge the battery "manually" with an external charger and then see what the voltage looks like after it sits 12-24 hours.

The battery may still be under warranty.
You aren't necessarily required to tell them the whole story.

 

not driving your car does not void the warranty

 

You might try a trickle charger on the 12V battery. They are around $50. I got mine at Costco, because with COVID, the car sits in the garage a lot. Costco sells two, I got this one: https://www.costco.com/battery-tender-4amp-smart-battery-charger-and-maintainer.product.100519712.html
It comes with two sets of cables, both fused, one with clamps, the other with eyelets which I attached to the battery, so it's very easy to plug in and out.

 

There was a hiccup somewhere -- This posted three times! Moderator, please get rid of this extra. Thanks!

 

True (hopefully), but @rmparker: the world’s BEST battery, left sitting in a car for 5 months (with the small-but-relentless drain any modern car imposes), will be dead-as-doornail.

Read up about how to keep batteries healthy, and proceed accordingly. Your current expectations are unrealistic; you might get a new battery under warranty, but with a similar scenario as before it'll be dead just as fast, and maybe next time the dealership will refuse your business.

 

NO, please don't.

What you are talking about is NOT a "trickle charger".
It is an automatic smart battery tender/maintainer.
BIG difference.

Tender = good.
Trickle charger = BAD.

 

here we go again...

 

I would first check if any of your battery terminals are loose.

All car batteries usually come with some sort of replacement warranty that is usually greater than one year. I would simply bring it back and have it exchanged depending on your purchase policy especially if you are still under warranty. Hopefully you still have your purchase receipt. Of course, some retailers would test the battery before taking it back in which case you will find out if the battery is dead.

 

I thought the only chargers you could use on those agm batteries were “intelligent trickle chargers”. They have a maintainer mode. They switch to maintaining when the battery is charged. Think motomaster makes one.

 

Thanks, I meant a smart battery tender/maintainer. (Us old guys still call them the wrong name, because these new-fangled things hadn't been invented yet.)