In article <x6WdnbJtQt9x31jQnZ2dnUVZ7qmdnZ2d@bt.com>, Brian wrote:
> That's because the USB standard specifies that a USB port on a PC
> supplies a maximum of 500mA, whereas mains chargers can supply
> 700mA, 1000mA or even more, depending on the charger design.
That's not how it works ... the amount of current drawn depends on the
device, not on the charger.
The USB2 spec says that a device may draw 100mA from any USB port and
may draw up to 500mA after negotiating with the host and being told that
it is OK to do so. This negotiation is supposed to allow devices that
don't have a high power availability (e.g. because they are running on
batteries themselves) to refuse to power/charge device with a high
current requirement ... and so to prevent devices from allowing
themselves to draw more current than can safely be supplied by the host.
This means that a device charging from a dumb wall wart or from a
powered-off PC can only draw 100mA, but a device charging from a smart
charger (capable of carrying out the negotiation) or a powered-on (and
mains powered) PC can draw 500mA and will charge *faster*.
These limits are changed to 150mA and 900mA for USB3.
But that's not the end of the story. The USB Battery Charging
Specification (of April 2009) added some new features and definitions.
In particular it added a convention that a dumb charger with high power
availability can indicate that it can supply a high charging current
without having to go through the negotiation process -- it effectively
shorts the data pins of the USB connector together. A modern device that
supports the USB Charging Specification can detect a dumb charger and
can draw a maximum of 1500mA from it. The charger supplied with such a
device will be capable of supplying 1500mA, and will connect the USB
data pins accordingly to announce this fact to the device.
> Some people are using smartphones as navigation devices in their
> cars and finding that, even though the phone is plugged into the
> charger (via the ciggie lighter or whatever) and so 'on charge'
> all the time, if it's a long journey (say a few hours) the battery
> is actually being run down because the phone is using more than the
> 500mA it's getting.
If the charger is an old one the device should not draw more than 100mA
(because the charger cannot carry out the negotiation that would allow
the device to draw more). If the charger and device support the USB
Charging Specification then the device can draw 1500mA -- which is
plenty (and which a car lighter socket should have no difficulty in
supplying). I would expect most modern smartphones to support the USB
charging spec, but it's very likely that the average Halfords car-
lighter USB charger won't ... so the device is only allowed to draw
USB Charging Spec: http://www.usb.org/developers/devcla...arging_1_1.zip
(not very easy reading!)
> Car chargers can be modified (by shorting out the data pins in the
> microUSB plug) so that the phone thinks it's on a mains charger ...
> ... but you'll never get any more than the 500mA out of your PC USB
And it would be dangerous (to your PC) to try!