I came across an interesting article today from Dr Gough, a tech nerd, and thought it was good enough to summarize here:

The USB specifications for power from a port vary from 100mA to 1.5A, and all the way up to 100W of power for USB Type-C. However, the cables and connectors used in a USB cable don’t always align with the power capabilities of the device being designed or charged. Most everyday USB charging cables are typically rated for about 1.8A of current, which is common for consumer-grade charging scenarios.

The 1.8A rating is largely based on safety limits related to resistive heating of the cable and connectors. This rating does not guarantee that your +5V at 1.5A setup will actually deliver maximum usable power to your device. The key point is that the cable-and-connector combination is primarily rated to handle heat safely, ensuring nothing melts or becomes a fire hazard. Most specifications go a step further and ensure the cable doesn’t even become noticeably warm to the human touch.

Every wire that isn’t a superconductor has some finite resistance. Put simply, electrical resistance turns energy into heat. Ohm’s Law tells us that E = IR, where E is voltage, I is current, and R is resistance. When power flows through a wire, current multiplied by resistance produces a voltage drop across that wire. That lost voltage becomes heat, meaning it never reaches your phone, tablet, or battery pack.

This also explains why some cables charge devices faster than others, even when using the same charger. Thicker conductors, better materials, and higher-quality connectors reduce resistance and therefore reduce energy loss. Cheap cables often cut corners on copper thickness and connector quality, which directly impacts charging efficiency.

I want to end this blog post with the above explanation because that’s the real takeaway. The more inefficient (or cheap) a cable is, the warmer it will get. If your iPhone cable is warm to the touch, it sucks. If the wire charging your power bank is warm, it sucks. Get a better cable. From what I can tell, there’s no meaningful rating posted on many of the low-cost cables you see on Amazon, so word to the wise: trust your tactile feedback.