Q. Is there an easy way to figure out if a USB port can charge a phone when the laptop is asleep or even powered off?
A. If only charging one mobile device from another had the same boring predictability of plugging a charger into a wall outlet. Instead, you have a few basic rules of USB charging that often get bent or broken — thanks to the different flavors of USB connectors shipped since the Universal Serial Bus's debut in the late '90s.
First, any laptop that's awake should charge any device plugged into it, even if the computer isn't plugged into the wall. This almost always works — but a grossly obsolete Dell running Windows XP (don't ask) failed to charge an iPad mini and only started the flow of electricity to an Android phone after taking a minute or so to recognize that device as a form of external storage.
If the laptop is asleep but plugged in, it should also still be able to charge a mobile gadget — that was the case with a 2012 MacBook Air and a 2011-vintage ThinkPad. But certain power-management settings in Windows can obstruct that as well in strange ways, and it's not hard to find reports from Mac users of equally mysterious charging failures.
What if the laptop is asleep and running on battery? Some Windows laptops will let you get away with that, courtesy of USB ports that continue to channel power under those conditions — and even if the computer's shut down.
The trick there is knowing what to look for, as there's no standard way to indicate this feature. For instance, that ThinkPad's sole "Always On" port has a yellow interior, while Dell PowerShare and Toshiba Sleep-and-Charge ports bear a tiny lightning-bolt icon (on a Mac, a similar logo identifies high-speed Thunderbolt ports).
You may also find that this option has been disabled in the laptop's BIOS startup menu.
Apple's laptops, in turn, can charge iOS devices while asleep and on battery — if the computer in question has at least half a charge left.
One extra complication comes from the different charging rates possible in USB 1.1 (the oldest, slowest kind), USB 2.0 (much faster, and the standard now) and USB 3 (faster still but relatively rare, identifiable on some PCs but not Macs by a blue interior). For example, pre-2007 Macs can't charge devices as quickly as those made since then.
Finally, you can find these different breeds of USB mixed on the same laptop without necessarily seeing them labeled as such.
I do not recommend leaving yourself to be surprised by a device's failure to charge from your laptop when you're on a trip — as I've done more than once. Instead, plug the phone or tablet into the laptop and verify that it charges when you expected it to.
(Once you have this straightened out, you can still find that the phone doesn't show up on the computer's desktop, leaving you unable to copy data to or from it over the USB connection; for example, one of my Thanksgiving weekend tech-support duties involved re-installing an Apple device driver to allow a copy of iTunes for Windows to talk to an iPhone. But that's a topic for another column.)
Tip: Check your laptop touchpad's settings
Most laptops, Windows or Mac, now default to interpreting a tap of their touchpad as a click of a mouse. And that setting drives me nuts — I find myself following links and selecting files when I didn't mean to. If that default or other touchpad settings have been bugging you too, you don't have to live with them.
In OS X, the Trackpad pane in the System Preferences app allows control of these options, with a "Tap to click" checkbox listed first. In Windows, you need to open the Control Panel, click the "Hardware and Sound" heading and then click "Mouse"; the rightmost tab in the Mouse window (sometimes labeled "Device Settings" and sometimes named after the touchpad itself, like "UltraNav") should allow you to tinker with these options.
In some cases, Windows users have also had to update third-party driver software to get access to this menu.