USB Charging, Computer vs. Line: What's the difference?
January 3, 2015 6:40 PM   Subscribe

My wife's UP band quit working. When she called tech support, they told her that they had found that some of the devices would quit accepting a charge from the plug-in, and only accept a charge via the computer. She chuckled, but tried charging from the computer and it worked. Can anyone explain to me what's going on here?
posted by gteffertz to Computers & Internet (5 answers total) 1 user marked this as a favorite
 
Probably a defect in the charger itself or if your using a different wire within the wire. The actual power shouldn't be any different.
posted by AlexiaSky at 6:43 PM on January 3, 2015


USB is an interesting little beastie. When you plug in a device into a computer, the host is supposed to send messages to a device to ask it questions about itself (what device ID is it, what device class(es) does it export and so on. In theory, a host is allowed to cut off power to a device if it (a) doesn't respond to these messages1 or (b) if it does respond and reports that it requires more power than the host is able to provide.

Although I'm not aware of any device doing this specifically, it is certainly within the realm of possibility that a device would choose to no longer accept power from a host that won't talk to it.

1While this is in the specification, very few if any hosts actually implement the cut off, opening up all manner of hare-brained devices that leech off the 5V USB line and nothing else.
posted by plinth at 6:53 PM on January 3, 2015


The power is different, the computer's USB ports supply less current, it's feasible that something is wrong with the charger or with the way the band recognizes the charger.
posted by Cosine at 9:58 PM on January 3, 2015


There are a bunch of USB devices that only charge from a computer. Playstation 3 controllers are notorious for this, I've run in to a few others. I can't explain why her phone would have been charging without a computer for awhile and now requires one, that sounds like a bug. But a plausible one.

USB charging is remarkably complex: see here, here, here, or here. In theory everything should just charge off the dumb 5V power line. Even that's subtle because there's different amounts of current that line can provide and some devices like iPads really only charge right if it can negotiate higher power. And then some devices insist on talking on the data lines in order to charge. As an added bonus, a data connection can be a security risk to either the charging device or the host computer, so occasionally you run into special cables designed to block the data lines. It's all a mess.

I've yet to see a single USB charger that can charge every USB device. I'm hopeful someone links one here, or at least links a better article about USB charging than I know of.
posted by Nelson at 7:37 AM on January 4, 2015


Response by poster: My wife says that the charger works with other devices. plinth may have it.
posted by gteffertz at 7:37 AM on January 4, 2015


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