12v battery dead, with maybe other problems

If your 12v is charging up you maybe OK. I am not sure of the sound you are hearing but it maybe a normal noise associated with traction battery control and other thing that runs automatically while car is parked. PRIME seem to have phantom draw on 12v while sitting, but 3-4 days for dead 12v is very short. Your 12v battery may be on the last leg. If you can't resurrect it, you may have to get a new battery.

Read-up on this thread to learn more about 12v AUX battery care on PRIME.
12v Battery Is Discharging, Re-Charge Now ! | Page 12 | PriusChat

 

it's probably an always on device that is starting to come up to operating voltage

 

Buzzing/humming should not be a problem, but I would be worried about clicking. My battery maintainer is 1.5 Amp. I am no electrical engineer, but could 6 AMP be too much? Also, if you're worried about the sound, disconnect the battery from the car (remove and wrap the negative terminal, after disconnecting the charger, of course) then resume charging it up...

 

if not, try disconnecting the 12v neg for a minute. if that doesn't work, you'll have to shut them off with an obd tool.

 

Not for a car battery. 10 amp would be fine. All modern chargers are automatic...they reduce the charge rate as the battery charge level rises.

Have you checked the liquid level inside the battery cells? You can remove the caps with a large coin and look inside. If the electrolyte level is below the tops of the grids it absolutely needs distilled water added. You can add the distilled water up to the indicator. Look into the access port of a cell. You'll see a kind of a tube from the top down a short distance with a slot in the side. After the battery is charged distilled water can be added to bring the level up to the bottom of that tube--you'll see the curvature of the meniscus when the liquid level reaches it. Add No More! (Topping up the water before charging might result in over-fill.) Keep the top of the battery clean and dry. Don't get dirt into the cells.

 

What kind of 6 amp charger? Old-school bought in the '70's, or an up-to-date "smart" charger?

There is an amperage limit in the 4th gen Owner's Manual, but do you think I could find it... IIRC it's saying 5 amp max??

 

Note that it's about a 45 amphour battery (I'm going by what's in a Gen 3, I don't actually know what they put in Gen 4), so if it was doornail dead and you're charging it at 6 amps, one hour is way early to check on it. If charging were 100% efficient you'd be looking at 7½ hours. Charging isn't 100% efficient, so a good 12 hours is more like it.