Not All USB Power Is Created Equal 240
jfruh writes "We've reached a point in our electronic lives where most of our gadgets draw power from a USB cable, and we have lots of USB ports to choose from — some of which live on other gadgets, some of which live on adapters that plug into your wall or car. But those ports supply wildly varying amounts of power, which can result in hours of difference in how long it takes your phone to charge. The Practical Meter, the product of a successful Kickstarter campaign, can help you figure out which power sources are going to juice up your gadgets the fastest."
kickstarter link (Score:2, Informative)
http://www.kickstarter.com/projects/david-toledo/the-practical-meter-know-your-power
Re:Or use what already exists (Score:5, Informative)
Basically an Ammeter (Score:5, Informative)
Re:Or use what already exists (Score:5, Informative)
Or one of these (it also passes through USB 3.0, which is nice):
http://www.amazon.com/Centech-USB-Power-Meter/dp/B00DAR4ITE [amazon.com]
This isn't new.
Re:Don't really see the market (Score:5, Informative)
What am I missing from this?
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
Re:Don't really see the market (Score:5, Informative)
Your Nexus 7 has runaway background processes. Otherwise there's no way it draws more than 500mA in standby - it would be empty after just a few hours. Check your battery stats to find the culprit...
My girlfriend's Nexus 7 charges just fine off of good old 500mA USB2.0 ports when it's in standby...
Re:won't help for Samsung note 2 (Score:5, Informative)
Indeed they do. If the charger says "I can supply 1.5A" but due to thin wires in the long, cheap cable that results in a significant voltage drop the device backs down to a lower level.
Re:Don't really see the market (Score:5, Informative)
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
Interesting. My wife has a Nexus 7 (2012 edition). It charges just fine (albeit relatively slowly) from 500mA USB chargers. It charges faster with the 2A charger that comes with it, but I've never had issues with it losing charge while plugged in to a standard charger.
How weak is your "weakly charging" USB port? Is it one on a keyboard or some other low-power accessory, or is it a port on the computer itself?
The Color and Tablet Nook devices have two different charge rates. If you use the official "USB" cable with the LED indicator in it, it charges at a 1A (2A?) rate. If you use a stock micro USB cable, it charges at the official 500ma rate. The decision is made by the Nook itself, based on info from extra pins that are in the custom cable.
Which (blankety-blank-censored-blank) is no longer available. And since the cables are no longer made or sold and since they were notoriously prone to fail means that I've been trickle-charging my unit for about a year now.
Moral of story: always check new toys for screwball cables before buying.
Re:Don't really see the market (Score:5, Informative)
The USB spec - ya know, that thing that every device carrying the USB logo is supposed to follow - permits a connected device to draw a maximum of 100 mA until it is properly recognized (enumerated) by the host. This is probably what the GP is referring to: 0.5 W of available power (less after conversion efficiency) isn't a whole lot for a device like a Nexus 7.
After being enumerated, the connected device can request higher current levels, up to 500 mA max. It isn't supposed to draw more unless the host permits it. For many modern portable electronics (e.g., smartphones) that have a 3-10 Whr battery, a 2.5-W maximum charge rate isn't much.
There are amendments to the spec that allow for greater power: in 2009, the spec created a Charging Downstream Port, which allows for up to 1.5 A from the host after enumeration; and the Dedicated Charging Port (DCP), which shorts the two data lines together and allows for 1.5 A charge power without enumeration.
Individual companies, such as Apple and Samsung, supply their own USB chargers that allow for even greater charge current, but do so in a way that technically violates the USB spec.
Re:won't help for Samsung note 2 (Score:2, Informative)
Voltage loss over such a cable is very real. At 0.14 mm^2 (AWG 26) you get 0.14 mOhm/m. For a 2m cable, 2 wires you end up with 0.56 Ohm. At 1.5 A that's a voltage drop of 0.84V.
Also note that such a cable is only rated for 0.36 A
Custom cables are almost always unnecessary (Score:4, Informative)
Which (blankety-blank-censored-blank) is no longer available. And since the cables are no longer made or sold and since they were notoriously prone to fail means that I've been trickle-charging my unit for about a year now.
Exactly why I avoid devices with weird custom cables whenever possible in consumer electronics. It's been my experience that unless a custom cable is so popular as to become a standard itself (like Apple's Lightning) that eventually you are going to run into a problem. Furthermore it adds to the cost of the device (custom cables = $) and it usually means that the company making the device had lazy and/or incompetent engineers. Now admittedly the USB spec is pretty flawed, particularly when it comes to power, but even so I've still seen lots of devices that could have used standard USB (or Firewire etc) had they taken the time to do so.
Now sometimes the standard needs to be updated. I think USB should be beefed up to handle up to 100 watts [computerworld.com] with all due haste.
Bear in mind that my day job is to run a company that makes custom cables. Think about that. I make a living off of custom cables, have the ability and equipment to make a copy of pretty much any cable, and I still think they are a bad idea for most consumer electronics.