Nearly every failed charging cable looks the same. The outer jacket is intact along its whole length, the plug is unmarked, and there is a soft, slightly swollen spot two to five centimetres behind the connector. Bend that spot and the cable stops charging, or charges only when held at one particular angle. The break is not in the middle where the cable is most often coiled. It is exactly where a hard part ends and a soft part begins.
That location is set by stiffness, not by weakness in the material. The connector shell is a rigid block of plastic over a metal insert. The cable is flexible. When you pull the plug out of a phone at an angle, or use a device while it charges, the rigid shell cannot bend, so all of the deflection has to happen in the first short length of cable leaving it. The sharper the transition from stiff to flexible, the smaller the length over which the bending is distributed, and the higher the strain in the copper and the jacket at that one line. A gradual transition spreads the same angle over several centimetres and lowers the strain everywhere.
What happens inside the jacket
The conductors are not a single wire; they are a bundle of very fine copper strands. Repeated bending work-hardens and fractures them one at a time. As strands break, the remaining ones carry more current, run slightly warmer, and become stiffer locally, which concentrates the bend further and breaks the next strand sooner. The failure is therefore a slow positive feedback, not a single event. By the time the cable works only when twisted, most of the bundle has already gone. That is also why the jacket often looks fine: the damage is inside, and the insulation is flexible enough to hide it.
The tapered rubber boot behind the plug exists for exactly this reason. It is called a strain relief, and its job is to make stiffness change gradually. A good one is long, tapers smoothly, and is flexible at its far end. A bad one is short and hard: it does not remove the stress concentration, it simply moves the break a few millimetres further down the cable. This is the same logic that makes a zipper fail at the bottom — the load finds the one place where the structure changes suddenly, and that place becomes the weakest point.
What reinforcement can and cannot fix
Braided jackets are often sold as the answer, and they do help with one thing: abrasion, tugging and the damage caused by being dragged across a desk or wrapped too tightly. But a braid does not change where the bend happens. If the braid terminates at a hard collar, it may even make the transition sharper. Aramid or steel strength members resist being pulled in a straight line, which is a different failure mode entirely. Neither addresses repeated small-angle bending at the connector.
The useful variable is bend radius. Hold the plug and let the cable hang; the length of the arc it forms before going straight is what matters. A longer, softer relief keeps that radius large. This is also why the only moving part of a power strip decides its lifetime: in both objects the lifetime is set by one repeated mechanical event, and everything else in the assembly is essentially passive. The plastic shell, the copper bar and a metre of cable are all capable of outliving the thing that moves.
There is a simple way to judge a cable before buying it. Hold the connector in one hand and bend the cable to ninety degrees a few centimetres out; feel whether the resistance builds smoothly or whether there is a distinct hard hinge at one line. Then repeat at the far end. A cable with the same gentle taper at both ends will usually outlast one that is reinforced only at the wall-plug side, because the phone end is the one that gets bent every day.
Most of the money in a cable goes into the conductors, the connector shell and the shielding. The part that determines how long you keep it costs a few cents of moulded rubber. That is the quiet economics of a very ordinary object: one small gradient in stiffness is worth more than all the extra copper.