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

[Maria Blackmore]

On Fri, 11 Apr 2003, Conor Daly wrote:

> Badly OT I know but...

Well .. it's technical ...

> 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.

Presumably, you mean a lithium ion battery?

> 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 love the little ironies in life


> 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.

Unlikely to be fatal, though it may not work as you expect, more below.

> I can desolder a USB socket from an old mobo and hook that up to the
> charger which can deliver up to 2.5A.

That's pretty chunky for 5V, the maximum current draw from a USB
port is 500 mA, so that should be just fine

> There are no drivers needed for the USB device so I presume it limits
> the current it will draw by itself.

If it's a lithiun ion battery then it will be carrying out the charging by
itself, yes.  The failure mode of a lithium ion battery that is being
improperly charged usually involves scattering red hot metal and chemicals
around the area, so all lithium ion batteries are "smart".

As for whether it will work, it's hard to say.  If you're using a charger
that supplies 5V it may well be expecting something on the other end to
react to it, and/or measure the current being drawn, voltage, etc to try
to form its own charging curve.  Basically what you need to do is present
5V and 0V to the USB device on the appropriate pins on its USB connector.
At this point it should theoretically just start to charge, however it may
or may not do this, depending on whether the charging relies on other
pre-requisites, such as being able to see a PC.  You'll need to make sure
it is just 5V and there's nothing trying to do anything clever with it,
and it'll probably be grateful of a nicely smoothed power supply.

In theory you should be able to use one of the generic variable voltage
power supplies with its output set to 4.5V, since the minimum acceptable
voltage on a USB connection is 4.4V.  You'll just need to make sure that
whatever supplies it can source 500 mA at least.

Have fun :)

Maria