One of the more persistent myths of charging is that using a USB port on a laptop is just as good as using a charger. After all, the charger has a USB port, and so does the laptop. Why should the laptop’s USB port be any different from the charger’s?
In reality, plugging a phone into your laptop is probably the slowest way to charge.
To get to the reasons why, we need a little background: a quick lesson on charge speeds and some basics on USB ports.
Computer USB port vs. wall charger
Charge speeds are measured by wattage. There are, however, two components to wattage. The formula is current (amps) multiplied by voltage (volts).
For example, the iPhone 6 and 7 come with an in-box charger that delivers 1 amp, and 5 volts. That means that the charger is 5 watts. Both of those phones, however, are capable of charging faster with a 2.4 amp, 5 volt charger. That charger is rated at 12 watts.
A laptop’s USB ports, however, at a minimum must provide 0.5 amps, which at 5 volts, would be just 2.5 watts, a fraction of the power.
Why laptop USB ports slow you down
Keep that math in mind as we discuss the basics of USB ports. Most USB cables and ports have four wires in them. A positive and negative for power, and a positive and negative for data.
When you plug your phone into a computer, the phone’s power management system senses the existence of data terminals. To avoid damaging the computer, the phone may ask for less current than it ideally needs. In many USB-A ports, that’s about 0.5 amps. USB 3.0 ports do a little better, but they still charge at roughly less than 1 amp. When charging with a laptop, a Samsung Galaxy S7 may only charge at 0.5 amps at 5 volts for 2.5 watts.
Chargers don’t have this problem for a very simple reason: They don’t have data lines.
When USB standards came out, there was a slight risk that the higher level of power would damage a computer when multiple devices were attached. That risk is much diminished now because computers have improved, but because the USB standard is meant to work anywhere, and for everyone, it hasn’t been changed. That standard is 2.5 watts, which is required on all USB ports.
So there you have it, everything you need to know about (not) charging a phone with a laptop. Don’t do it if you have anywhere to be soon.