Jumping 12V Battery direct from Battery

That is not a good diagnostic tool, as it doesn't give you a true reading of the battery itself.

About all you can do is drive it and see what happens.
OR
You can leave it sit for a couple of days and see how long it takes the charger to call it "full" again.
If it takes more than an hour, you need to look for a new battery.

 

I have no idea.
The first attempt probably put a little charge into it.
The second attempt might have worked either way.

 

Just editorial: for 2nd gen the amperage mentioned in the Owner's Manual is 3.5. As to what that number means, I'm keeping shtumph.

 

Haha, he forgot “there’s no place like home”. I think you have to repeat it twice. As a disclaimer night not work without the cute dog. Or was that another story?....

 

just sharing some experience I'm having today with a dead battery.

• We think it was a dome light left on for about 18 hours. 2008 Prius
• My 4 amp charger bought today at Costco did not seem to work and made the recipient car click fairly loudly without stop until it stopped showing it as charging the vehicle.
  • Dome lights could light very dimly during the Prius clicking part.
  • After we tried starting it (prematurely in my opinion) the charger would not resume showing it as charging.
  • The charger in my opinion was always fooled by the Prius. The clicking phase lit a 80% full light on this smart charger (AMG capable) but I'm pretty sure this battery went way down close to zero.
  • We are having success getting the dashboard to light up and other Prius doodads to whiz and click with a Prius to Prius charge attempt. I'm giving it an hour.
• It is most definitely 100% possible to safely jump or charge a Prius 12 volt battery, provided ...
  • you use the jumper port in the front of the car that is located in the fuse box..
  • and the cables on the powering car or charger are also connected correctly.
  • Prius to Prius works. They share the same issues and are compatible. You can jump start a Prius with a Prius because they are both Prius. I recommend letting recipient charge up for a while, then disconnect all the cables before attempting to start the recipient Prius. half hour, maybe an hour, then disconnect All.the.cables. and attempt starting the previously uncharged car.
  • And recipient car might still need a new battery even if a jump works.
• "Smart Chargers"- Chargers that are smart.
  • My understanding is the term smart in these kind of chargers means they are supposed to stop sending electricity down the wire when there is no current. Smart means they are not supposed to be live wires unless they detect a BATTERY is connected.
• Chargers that are good for Prius
  • Prius 12v battery is different than the old ones that cars had for a long time. They have an AGM battery “absorbed glass mat”.
  • Chargers should have a special charging method for AGM batteries.
    • It could be a button you press to select between AGM and non-AGM or
    • It automatic (which is different than 'smart') and auto detect AGM, but the charger should have an indicator light about AGM to let you know it is charging according to AGM Battery requirements.
    • You definitely want AGM capable in a charger.

Update: The charge didn't work after an hour. With the cables disconnected the disabled car wouldn't even light the interior lights. A jump start did work, and we probably need a new 12v battery.

 

Not sure "probably" is the right word.

You have gained some good knowledge and experience but it REALLY isn't that complicated.

Note: It really is NOT necessary to have a charger with an "AGM" mode. The difference is very small and not significant in most automobile applications.