USB-C fast charging: why the cable slows you down more often than the charger
You buy a 65 watt fast charger, plug it in with a cable you had lying around, and your laptop charges slower than expected. Nine times out of ten that is the cable.
Same plug, different insides
USB-C is a shape, not a promise. All sorts of things sit behind the same connector. The power conductors differ in thickness, and that is the problem: a thin conductor handles less current before it heats up.
Roughly speaking there are three kinds:
- Cables up to 60 watts (3 amps at 20 volts). This is the standard and most cables you get for free are these.
- Cables up to 100 watts (5 amps). These carry a chip that tells the charger more current is allowed.
- Cables up to 240 watts, since the newer USB-PD revision. Not many devices ask for this yet.
Without that chip a charger stops at 60 watts even though it can do more. It does not know whether the cable can cope and sensibly backs off.
Data and power are two things
Another confusing point: a cable can deliver 100 watts while only reaching USB 2.0 speed for data. That is not a fault, they are different conductors. To connect a monitor or a fast drive you need a cable that separately states USB 3.2 or Thunderbolt. For charging alone it is irrelevant.
Length counts
The longer the cable, the more voltage drop. At two metres and 100 watts you notice it. If you want a long cable at full power the conductors have to be thicker, and you feel that: it is noticeably stiffer.
Practical
- Check the cable or packaging for wattage or amperage. If nothing is stated, assume 60 watts.
- For a phone 60 watts is plenty, it rarely draws more than 30.
- For a laptop of 65 watts or more you need a 100W cable, otherwise it charges slowly or only in sleep.
- If a cable gets warm while charging, do not use it for demanding devices. Heat is loss.
A set of 100W USB-C fast charging cables is mainly useful when you want one lying everywhere, because the missing cable is always the one you need.
The simplest advice
Buy a few good cables once and throw out the pile in the drawer. Most charging problems disappear with that, and at least you know what you are holding.