Where Is The 12V Battery On A Gen5 HEV?

3rd gen battery is under the hatch floor.

The pic you attached (from gen 5 owner's manual?) is highlighting the jump start point. The aforementioned stud-and-nut that feeds 12 volt into the fusebox is elsewhere in that pic. Have a look at the video I posted. Again, the video is of gen 3 underhood fuse box, but gen 5 may be similar.

For an onboard charger running the 120 volt power cord out through the front grill seems sensible. Parked outside, you're able to leave the hood latched, are just running an extension cord out to it.

If on the other hand you were running a quick connect out to an external charger, there could be disastrous vandalism: it's basically a two-pronged connector, with the prongs connected to neg and pos battery posts. Someone putting a jumper wire between the two pins would short the battery. Even heavy rain might short it.

 

I don’t think permanently mounting it will help you or save you time. You still need to open the hood, carefully route the AC cord through a gap over the headlights, plug it in, and then close the hood, which will take quite a bit time. If the car is rarely used, it is less reason to mount it permanently. That is what I do when I go on a vacation. I close the hood over the AC cord routed through the gap in the corner of the headlight. That way, if someone breaks into the garage, the car can’t be vandalized or its parts stolen, or animals and dust can’t get into the engine compartment.

Also, in Gen 5, the terminals under the hood will be safer to use than the battery terminals. If you want to use the repair mode, don’t forget to disconnect the negative terminal of the battery in the trunk.

 

To try to partially rehabilitate a battery that saw a deep discharge. Lead–acid batteries degrade if the state of charge drops below about 50%.