Here's Why Your Car's USB Port Charges Your Phone So Slowly

You can use your car's USB port to charge your phone, but you've probably noticed it's a painfully slow process. If your phone isn't charging at all, the car's USB port may not be working. You can fix that, but if your phone is just charging slowly, there's not much you can do.

Charging speed depends on the volts, amps, and watts delivered by the charger. Modern phone chargers are typically rated for five to nine volts and around two amps. The other factor is the power source; your wall outlet at home can put out up to 120 volts at 15 to 20 amps, which is more than enough to support fast charging. By comparison, some cars put out just half an amp through their USB ports.

Speeds have improved somewhat ever since cars started adopting USB-C technology, which can charge devices up to 20 times faster than USB-A under the right conditions. Kelley Blue Book provides a list of cars with USB-C chargers. However, most in-car USB ports still use the USB 2.0 standard, which is limited to 2.5 watts of output. Since modern phones can draw 20 watts or more, it's clear why an in-car USB port can't keep up with a charger plugged into a wall outlet.

Your phone charges slowly because your car's USB port has limited power output. But you can skip the USB port altogether and plug a fast-charging USB-C adapter into the 12-volt power socket on your dashboard. As long as you have a USB cable that supports fast charging, this adapter can deliver up to 75 watts to your phone, assuming it supports that level of fast charging.

Something to note is that fast-charging can hurt your phone battery due to the heat it generates. The impact is negligible on its own, but fast-charging while also running intensive tasks can make your phone dangerously hot. And one of the most intensive tasks your phone can do is navigation. For example, Android Auto might be killing your phone battery, especially if you're using it in a hot car while your phone is plugged into a fast-charging adapter. Having access to fast-charging in your car may be a relief, but it's not something to use all the time.