Quote:
Originally Posted by dwig
... Back to chargers, there is another issue to consider. With most tablets and phones the "data" lines (USB2 uses 2 wires for power and 2 wires + the power ground wire for data) are often used to signal the device that a charger is present. There is a USB standard for this, but many devices use their own proprietary system.
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True in many cases (especially full-sized USB connectors), but many devices also use a cheaper approach where the fifth pin in the microUSB connector is pulled up with a specific value resistor that determines which function any particular microUSB cable or device supports. In the case of quick charging, resistors are commonly used to tell the charger the device can charge at a much higher current. Without such resistors, the charger defaults to typically 500mA to act like a PC USB jack (
except for apple, which refuses to work for marketing reasons). That fifth (ID) pin is also used in USB OTG connectors, to tell the e-reader or cellphone to act as a host and to suppy power to the microUSB connector. Other than OTG, resistor values and functions are proprietary and vary by manufacturer, so using the wrong cable or charger could have unsatisfactory results (in most cases just taking longer to charge, or failure to function as expected).
Here are some resistor "standards" for full-size USB (most chargers, as you described) used by various manufacturers:
https://en.wikipedia.org/wiki/USB_On-The-Go
"Three additional ID pin states are defined at the nominal resistance values of 124 kΩ, 68 kΩ, and 36.5 kΩ, with respect to the ground pin. These permit the device to work with USB Accessory Charger Adapters that allows the OTG device to be attached to both a charger and another device simultaneously."
More fun with microUSB ID resistors:
http://forum.xda-developers.com/showthread.php?t=820275
Resistors are also used to select data speeds. With no real standards regarding many resistor-selected special USB functions, it is all rather confusing. Best to use the charger that goes with the device, and sometimes the matching cable as well. However, some say specialized chargers are just a
waste of money.
And in some cases, device features are selected via digital signalling over the USB data lines, instead of using fixed resistors (e.g.
USB Power Delivery signaling).
And if you think that is complicated, my Samsung Galaxy smartphone has ELEVEN pins in its microUSB jack (which include 4K video out, which did not work on my first non-Samsung 4K-compatible cable even though it claimed such support).
I think an XKCD cartoon explains this situation pretty well:
How standards proliferate.
Now, getting back to the ORIGINAL question in the first post, even if your charger tries to charge a battery too quickly, they have built in protection circuits to prevent a fire in your pocket, so it will generally not harm them other than shortening their life a bit. But still, better not to do that, just in case.
But beware of cheap dollar-store chargers -- some of those put out 9v or even 12v (or worse) without checking the resistor values on the cable. Some devices will not tolerate that.
Thus ends another exciting installment of the "
little professor" lecture series. Stay tuned for more.
