Do I need to disconnect 12v battery?

The C battery is not larger than the Gen 2 battery. I would continue to disconnect. If you want to do it right: amazon.com/dp/B015FN2YQY

 

If this is a once-in-a-blue-moon thing, and urgent, I would just pull the clamp off the negative battery post and isolate it.

 

is the c battery difficult to get to?

 

Its not hard but is a pain in the @ss. As the Prius c battery is under the rear seat passenger side

 

that's what i thought. if i had to do it regularly, i would add a connector.

 

Yes there is enough space.

 

What I would do is hook up a smart charger, one that can be left on indefinitely.

Our car typically sits idle a day, sometimes two or three. Parasitic draw starts to take a toll; I hook up a CTEK 4.3 maybe every couple of weeks.

(This presumes you've got secure parking and a 120 volt plug-in.)

 

firstly, i think they added the jump point under the hood on the c by 2014, so start there. if not, i would run the negative cable to the hatch area, and use that item jumbo recommended, then back to ground. i think they alight also make quick disconnects, but not sure.

 

I know the Gen 2 and V and will assume the C is similar.

All sorts of connections are made off the positive Post of the battery but the negative Post just goes to the frame. If you hook the frame end to the Switch and add another small cable between the switch and the frame, you can isolate the battery.
amazon.com/Arnold-16-Inch-Battery-Cable-Set/dp/B003RCJB62

I hope that helps. (The image I found does not show where the negative cable connects to the frame)

 

Looks like the neg. cable is grounded right down at the battery base. Might be an isolator switch could work there, with some mod of the back seat base. One of these:



Maybe just inboard the battery there'd be more space. That battery location seems better for an isolator switch than the 2nd/3rd gen Prius with battery at the back/right corner under hatch floor.