Field wiring is the portion of the system that connects the central control or power source to the outlying loads or utilization equipment.

Like I said earlier, soldering works, but well twisted wire ends that are held captive by heat shrink will never fracture, never come apart, [never show a micro-ohm's worth of difference at 5 volts and 200 mah]. The heat shrink exerts alot of pressure on the twisted splice and locks it in. It works perfectly.
If there was a problem with this method, it would have shown itself to me about 20 years ago.
Obviously, solder has to be used by the hobbyist to make reliable battery packs, but I make sure to immobilize the wire after the solder joint so it doesn't fracture.