Why won't the computer's USB port charge my phone?

Today’s smartphones are like milkmen: they’re shattered by lunchtime, which means you’ll often need to plug the phone in for a quick charge at your desk. But why is the phone not charging – or charging very slowly – when you plug it into your PC or laptop’s USB port?

This basically boils down to power. Although the USB port on your computer looks the same as the one on your phone charger, the current output is very different.

Some older USB ports on computers may deliver little or no power. Some companies disable USB ports on their computers because they don’t want staff plugging in disk drives and taking home sensitive information. And even fully functioning USB ports may not have enough juice to top up your phone as quickly as you’re used to.

Many of today’s posher phones – and even more economical ones – use a system called Fast Charge that blasts the battery from empty to half-charged in as little as ten or 20 minutes. But for that you need the charger the phone is supplied with; you won’t get that kind of performance by plugging your phone into a computer’s USB port.

If your phone isn’t charging at all from a computer’s USB port, and you’re sure the port is working, then there are a few things to check. First, the computer normally has to be powered on (we’ll come back to this in a second). Computers that have put themselves to sleep will often cut power to USB ports, which might explain why you left your phone on charge while you went to lunch and it’s only added 2% by the time you got back to your desk.

You can fiddle with your computer’s power management settings (search for ‘power’ in the Windows 10 start menu to access the relevant settings), although you might reflect that there are more environmentally friendly ways to charge a phone than keeping a desktop computer running.

Some laptops do allow you to charge your phone from the laptop’s battery, even when the lid of the device is shut and the computer is on standby. Look for a little symbol with a lightning squiggle inside a battery next to the port, which indicates that it can be used for such purposes. Obviously, your laptop battery will need to have sufficient charge if it’s not connected to the main supply.

This so-called Always-on USB feature sometimes needs to be switched on in your computer’s settings. The utilities that computer manufacturers supply with their systems (such as Lenovo’s Vantage) will normally have an option to turn this feature on or off.