Even brand new 12V batteries are not

prius is a poor charger, more of a maintainer. try charging it up, then see what she holds at.

 

First step to replacing a battery is making sure the new one is fully charged. Not just trickle charged, but fully charged with a real charger! Seems like this did not happen for you?

 

What's your usage pattern like ?
In addition to never being fully charged, your new battery might be defective.
How are you getting a warranty battery on an 18 year old car ??

Except for that 11.8, your numbers look about right.....or at least acceptable.

 

Have you ever drained the battery in the three years since installation. If it was fully discharged for any period of time, The lead plates have sulfide crystals and the battery is no longer fully capable. A good battery maintainer may be able to break up the sulfide crystals and restore the battery to usefulness.

JeffD

 

No, the inverter is always on at a slow charging status whenever your Prius in in "Ready" mode. It takes a long time to fully charge a depleted battery with a constant voltage charging system like the Prius. Once you left the battery depleted for a "few weeks" your battery was toast.

JeffD

 

An agm battery can be charged at 14.9 vdc if needed. The higher the charge voltage the higher the amps going into the battery. 14.1 vdc charge voltage means the battery needs more charge; 13.8 vdc charge voltage means it is close to fully charged. On my car, 13.4 vdc while driving (not Park) is full. So your battery is not fully charged if all you see is 14.1 or higher in Ready and driving.

1. If you garage your car or have a nearby ac outlet, a four stage battery maintainer like a Battery Tender Jr ($29) is a good idea for a vehicle that is not used much. This model comes with a hardwired cable to your battery or front jump point and a quick connect where you don't have to open the hatch or hood to use.

https://bit.ly/BatteryTenderJrWalmart

But there are other possible issues the informed owner should check.

2. One is the battery is no longer as good as it was new. Load check it at an autosupply.

3. Realize all modern cars drain the battery anytime they are off. Parasitic draws of 15 ma to 30 ma are reasonable with a Prius. Often aftermarket accessories or devices plugged into the ODB2 connector draw 50 ma - 1000 ma continuously even when "off". Occasionally lights are on somewhere, such as the hatch light. Sometimes standard modules causes excessive off state draw.

Any excessive draw over 30 ma will quickly reduce the 12v batteries stored charge. A map light can discharge a battery to 10 v overnight, smaller parasitic draws could easily discharge your battery to 11.8v overnight. What is your car's parasitic draw?

4. Keyfobs left next to the car sometimes cause higher than desired discharges for the first four hours.

 

What he said.
No mystery here.
The onboard system was not designed to completely recharge a dead battery.
That's what a battery charger is for.

 

I would check the connections, including the ground strap to the body, then not worry about it as the 12v battery is under warranty.

Either the prius is "OFF" or "READY", anything else is asking for trouble.

How many hours per week are you currently driving?

 

0.8 amp of 12v power is not enough to charge although it might accommodate a normal parasitic draw. Excessive parasitic draws come from many sources, we have seen things like the power steering module draw 900 ma continuously. 0.8 amp is 800 ma of peak power which is not continuous, eg you get no power at night or on overcast days. So measuring parasitic draw is the way to go. Having the battery load check is essential, if it is weak no amount of charging will help.