Kinda an odd problem about my 12V battery-cables

The other end of that heavy cable is connected as the supply source for the underhood fuse box in the upper driverside of the engine compartment. There is no real slack to be had.
You'll either need to get the crimp fitting removed, opened up and recrimped/soldered, OR do what some others have done and find a salvage yard that has a gen 2, cut that cable several inches back and crimp/splice the new piece of 'yard' cable onto yours. You don't want to make a loop in the wire, so you'll need to trim it as needed to get the correct original length. Otherwise it's just a pain to stuff it in place.

 

but, for that size wire and ampacity, is less common, and requires the heavy crimp tools they have at your neighborhood independent auto electric shoppe.

I would probably try to get the last foot or so of that wire out of a salvage car, then take it to my neighborhood auto electric shoppe and have them make it up. They'd charge me less than the cost of a heavy-nice-person tool I might never use again.

 

If it goes to Ready with the short salvage battery wire jammed into the remaining vehicle harness it might work for a short time. A battery that could not staying charged before has no chance with a battery cable rigged in that manner.

To properly repair using a salvage pigtail (piece) it should be crimped or lugged at the spliced connection.

Crimp sleeve for wires (sized for wire diameter - typically 4-6 awg with automotive stranded)



Lug Butt Splice (tighten with allen wrench and insulate)



Amazon Butt Splices



Once you splice the wires together, you still need to do something about your battery. It is likely weak and needs testing and replacement as indicated.

Again, a good battery adequately charged won't freeze. How many of your neighbors are removing their battery every night? Some will attach a battery maintainer as I have when in Calgary during the winter. Many do nothing and still crank their high amp starters.

A Prius 12v battery does not have to supply large currents and is in the cabin rather than under the hood. After achieving Ready, a weak battery can initially charge by drawing 60-80a from the hybrid system. A loose, "jammed" connection won't do it and is likely to fall out and short the battery. At best you can be stranded somewhere.

 

The markings on that splice identify the not one but two distinct special tools and dies to be used to properly crimp it onto the wire.

Your neighborhood independent auto electric shoppe already has the right tools and dies to do that, and when they're done you'll have a properly all-around-crimped and insulated connection, more reliable than the set-screw butt connectors, and they'll probably be done in a couple minutes and you'll be on your way for a nominal charge.