2020 Prius Prime - Dead battery needs charging

It's a conventional battery, use the 12A setting.
Check the electrolyte levels using a loonie to remove the caps. None of the liquid should be below the plates inside. Top-off with distilled water to the bottom of the plastic tube, if needed.
Charge for an hour or two, then disconnect and see if it starts.

Hope this helps....

FYI: the 75A setting is for jump-starting the car. DO NOT use this function, since the car will probably die as soon as you disconnect it. Best to simply recharge the battery and go from there. The high amperage across your aux. battery may damage your electronics (feed-back loop).

 

I had 4 dead 12v ICE batteries that each lasted about 18 months and all had a bad cell, and dealer tech's were mystified. Real culprit is the ICE battery not being charged when you charge the traction battery and being successful in not using Charge Mode and being able to live off plugging it in. So, the more EV Mode your able to achieve because of keeping the skinny blue line on power diagram less than the fat blue line, coasting, short trips, anticipating and coasting over hills, and not running in Charge Mode, the more likely your ICE battery will die sooner without ICE TLC. I'm getting over 100 MPG per dashboard, and fill gas tank monthly, and it kills the ICE Battery. SO as numerous others had said, check battery with meter occasionally or use a Cigarette lighter USB charging device that tells you battery voltage, and charge it, or leave a solar panel plugged into cigarette lighter and parked in the sun. Or rig-up your own low voltage alarm.
This fatal design oversight is not unique to just the Toyota plug-in.

 

Another reason not to go to the 75A is the 12V battery has less capacity than a "normal car's" 12V battery. The car doesn't use the 12V battery (much) during normal operations, so they only put in a smaller-sized one. You could damage it further with the rapid charge rate.

The 12V battery started failing not too long after I got my 2019 Prime. I've got a 6A charger (I bought in the 1970s), and tried to charge the battery. It seemed to work, but then my wife needed a "jump" to get the car started to go home from work a couple days later. That's when I decided the battery was gone, and we needed to get a new one.

I checked out the prices, capacities, and warranties on the replacement batteries from all of the auto parts stores around here. Just for kicks, I called the local Toyota dealer - it was surprising that they had the best price, warranty, and they also had one in stock too. Go figure.