12 V Battery Dead?

That is interesting as I am having the same (similar) problem with a 2018 Prius C out in South Haldey, MA (I used to live in the Lemon Grove part of San Diego so I totally know where the El Cajon Toyota dealer is.... but I'm too far away out here for them to help me so I have to bug you (Hope you don't mind).

When you charged your battery, did you connect the charger directly to the battery under the rear passenger seat or did you continue to use the connection point under the hood? Also, what did you use for a negative under the hood (I could not find one as depicted in the book that came with the car so I used the engine block, then the frame, then a mount - car still would not jump start or charge.)

 

you can use most any bolt under the hood. the charger will tell you if you have a proper connection.

ctek is one good brand, schumacher another. there are lots of them on amazon, etc. and they are inexpensive.
just hook it up, plug it in, and leave it on except when using the car. the maintainer takes care of everything.
you can even get a harness for the battery, if you want a quick way to connect/disconnect

 

Battery Tender is another popular brand and easy to find. I have A Schmacher charger and a Battery Tender.

 

For the negative charger cable, bare metal on the engine is a very good bet. Most any substantial bolt in the body too.

A cheap continuity tester like this:



Is good, if you want to verify the prospective negative connection point (aka "ground") is good. Connect the tester's clamp to positive, then test various metal with the probe. If the light in the handle lights up, it's good.