How much driving to fully charge the 12V?

Far far too much to bother to drive just to charge. Get a battery tender and run it overnight. The tender may even be cheaper than the fuel needed for a long-enough highway drive.

The charging system voltage steps down when the battery gets closer to full charge, at least in my Gen3. If your Gen2 isn't multi-stage (or not enough stages), then it's charging voltage and rate are set so low that few drivers will ever run into an overcharge situation. Instead, pandemic 'stay at home' practices are causing a lot of undercharge problems.

Your drives are much too short to approach full charge. And your waits after driving and before measuring voltage are much too short to allow the battery to adequately relax from the charging process and possibly give more useful stable readings.

If the battery isn't freshly off the factory floor, then it just isn't 45AH anymore.

You are putting way too much effort into this, without taking into account the vagaries of these batteries. Get a battery tender and leave it hooked up overnight.

 

On my cars I've used a solar battery maintainer mounted in the back window and wired directly to the battery. The charger did not recharge the battery. It just made up for the very small drain caused by the electronics.

 

I once checked out a solar battery maintainer, found by a friend, that lit up its 'working' LED even when its output was well under 12V. It needed only enough PV output voltage to light up that LED.

It was capable of doing some charging in excellent conditions, but was fooling the user the rest of the time. Behind glass, at a poor angle, in the Seattle-area climate, 'the rest of the time' was a large majority of the time.

 

put the charger on it, thats far too low

 

That is low for a brand new battery. You need to have charged that up before installing. A brand new AGM should be 13.2V after the surface charge has dissipated.

No. It is a safety mechanism to vent off explosive gases in the unlikely event of a catastrophic over-charging event.

My first impression was the round brown sticker contained the date code – March 2021.

 

No it shouldn't. That is the charging float voltage.
The battery itself should never be above 12.8 after resting a while.

 

Those voltages seem a bit low, across the board.
That probably means that your new battery needs a good manual charging.
And that the charging voltage with the car running needs to be checked too.

The "vent" is not open unless or until the pressure inside the battery gets too high.
It is a safety valve. In normal operation, it does nothing.

 

Prius AGM battery is designed to handle deep discharges. Battery tender junior not designed to charge discharged batteries.

Should have gotten a 7-10 amp smart charger w/AGM setting and fully charged the old battery properly to get a good idea if it was indeed bad or use a battery tester as Mendel suggested.

Majority of name brand smart chargers are made in China.

If the Prius is not driven for 3-4 days, disable the smart key button under the steering column.

 

Not good advice. The tender should handle it IF it is not too far gone; just takes a LONG time.

AND Toyota recommends not charging at a rate over 4 amps.

 

Then why does a Toyota TSB mention a 10 amp charger setting that dealers have been using for a number of years.



The latest Toyota approved charger also outputs 10 amp. Note: Output Current: 10A CONTINUOUS

Product - Toyota Approved Equipment


The OP's Battery Tender Junior charges at .750 amp instead of the 2 amp setting that Toyota recommends against because it would take too long , wonder how long .750 amp would take

 

2nd gen owners manual caps it even lower, 3.5 amp IIRC.

 

Optima AGM battery manufacturer page also specs 10 amp max. Subsection charging Optima batteries

OPTIMA® Support - Charging, Maintenance, Storage & More | OPTIMA Batteries

OPTIMA YELLOWTOP® CHARGING INFORMATION
The following charging methods are recommended to ensure a long battery life. Always use a voltage-regulated charger, with voltage limits set as described below.

YELLOWTOP Type: D51 & D51R - D35 – DS46B24R - D75/25 - D34 – D34/78 - D27F - H6 - H7
These batteries are dual-purpose. They are designed for engine start and cyclic applications and for use in vehicles with large accessory loads.

Recommended charging information:

• 13.65 to 15.0 volts, no amperage limit.
  
• 13.8 to 15.0 volts, 10 amps maximum, for approximately six to twelve hours.
  
• 14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below one amp, finish with two-amp constant current for one hour.
  
• Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below one amp.
  
• 13.2 to 13.8 volts, one amp maximum current, time indefinite (at lower voltage).
• Strictly adhere to all limits.

 

Sam, you need to get educated in these newer technologies.

AGM cells are 2.2V. There are 6 cells per battery. That is 13.2V

One can not fix stupid, but one should not encourage it either.

That was written 2004 - 2009. Technology changes and improves. Toyota has the a right to change it's guidance.

I have also read other battery manufacturers recommend 10A chargers for their batteries.

 

I had this discussion with Spade in this thread with similar documents but he still spouts his standard 4 amp line.

No start, battery dead? | PriusChat

So yes Spade, you are absolutely wrong. A Toyota service bulletin for dealers says so, and also Toyota approved charging equipment also says Spade is WRONG. How do you like them apples SAM.

I see I'm having trouble fixing ............so I'm trying not to encourage it. I'm giving Sam a new name 4 Amp Sam.

Not good advice? Also one more thing, Optima Yellow Top battery manufacturer also says 10 AMPS.

 

But some folks just want to argue.

 

No they are not.
YOU need to review what you think you know.

 

OYT is not an OEM part.

And ANY common rechargable battery will generally have a longer life if it is charged and discharged more slowly.

You are welcome to your own opinion but not your own "facts".