12 volt battery... what am I missing?

Batteries in general cost money because there is an upsale point in the market place. The prius does not have an alternator and requires a second battery to be present to give you convience power and run computers in the car. The car its self draw 30 amps with all the lights and ecus going. So you need a decent reserve to keep the car running or your gonna lose mpgs to electrical generation needs. You can put a lawn battery in but its not gonna last 6 months before it goes flat from deep cycling.

Posted via the PriusChat mobile app.

 

see my posts #11 and 12. perhaps you need more help than can be provided, what problems are you having?

 

honestly, sounds like the Seller misrepresented the battery's condition. it could have been well past its useful life. Being inside the car, doesn't get much road-rash weathering

 

Did you get an AGM for 30$? Wow... What capacity?

The Prius2 battery is an AGM, but it fails after a few full drains. So it is not a true deep cycle, at least that is the general consensus here on the forum. Replacing it with a normale lead-acid or a gel can be tricky as the charging regime for those are slightly different. Then again, when researching this years back, I came across seemingly informed websites claiming opposite 'facts' (e.g. that a gel should NEVER be charged with 14.4V because it is to high, another claiming to charge a gel with 14.7V).

I personally replaced my 38Ah (?) original Prius battery when it was 6-7 years old with a brandless 55Ah AGM deep cycle version (also different battery-posts, several threads on how to do this) and I expect this battery to easily survive for a decade unless I fully drain it.
It had no problem starting the computers after 9 weeks (big trip), drove across the parkinglot to be close to the powersocket and charged it just to be sure.