[Techtalk] [OT] charging USB device without a PC

Almut Behrens almut-behrens at gmx.net
Sat Apr 12 03:09:35 EST 2003


On Fri, Apr 11, 2003 at 10:01:54AM +0100, Conor Daly wrote:
> 
> Recently got a USB flash rom/mp3 player combo
> (http://www.marx-computers.com).  Lovely _little_ device, 128Mb flash with
> MP3 playing.  Uses a lithium battery that charges off the USB port.  Now,
> I'm going away for a few days and it seems just _wrong_ to have to lug a
> 10kg PC with me just to charge a 30g mp3 player.

:-))

> I have a 5Vdc charger
> (from another device) and I'm just wondering if it would be fatal to the
> USB device to attempt to charge it from that.  I can desolder a USB socket
> from an old mobo and hook that up to the charger which can deliver up to
> 2.5A.  There are no drivers needed for the USB device so I presume it
> limits the current it will draw by itself.

I think you should not have to fear anything.

Of course, theoretically, one could implement the current regulation
partially in software, but I couldn't think of a decent reason why a
manufacturer would want to do this. There would be about zero savings
in production costs, but an added risk and instability due to potential
software problems. What if the driver hangs unexpectedly, so the
current would not get reduced and shutoff as required? -- this could
seriously damage the battery or even cause physical hazard (explosion).
For maximum reliability and safety, one would probably want to
integrate that delicate control circuitry directly into the battery or
device hardware. Also, you mentioned that there are no special drivers,
which makes it even more unlikely to be realised in software ;)

However, as you can never be 100% sure, I would just try to charge
the battery in a kind of experimental setup, and keep an eye on the
temperature of the device, in particular during the final stages.
Batteries do not explode out of thin air from one minute to the next,
that's to say, if the whole thing should really turn unexpectedly hot,
there would still be enough time to switch off the power. A slight
warming-up could also happen under normal circumstances -- so to get
an idea of what is normal, just watch the battery temperature during
a regular charging cycle via the PC.

The 2.5A of the charger should be more than sufficient, as the maximum
current a USB port delivers over a longer period of time is 500mA
(over-current protection shutdown is supposed to trigger at 5A).

So, just connect the right pins (+5V to pin 1 = VBUS = red wire, GND to
pin 4), and everything should be fine...

Socket pinouts and other details (more than you ever wanted to know)
can be found in the USB Specification available in the developer
section on www.usb.org.

Happy mp3-ing,
Almut


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