I wanted to report the results of measurements I made with a breadboarded charging circuit - in case this might be relevant to others who may be having problems.
But first, a quick reminder. When connected to an AC or car charger, the Clip behaves normally, meaning that the screen goes dark after 15 seconds, and you can play songs, etc., just as if it wasn’t plugged it. However, the Clip does actually have to be turned ON to charge. So, you need to turn Auto Power and Sleep OFF when charging. Otherwise, the Clip will shut down at the end of the Auto Power or Sleep period, and charging will stop.
Anyway, in March, 2007, a new USB standard was issued for charging through a USB port. The standard says that a “dedicated charger”, which is just a non-intelligent power source provided through a USB socket, should have its Data lines shorted together. The Data lines are the two middle pins of the socket. A connecting device, upon detecting that these two pins are shorted together, will know it need not try to negotiate a formal connection, and can just proceed to draw current. Older chargers, and possibly newer proprietary USB chargers for other devices, may not have the Data lines shorted.
My 1 GB Clip with .20A firmware, turned ON but with audio playback paused and the screen allowed to go dark, and with the Data pins of the charger shorted together, draws 154 ma. from a 5VDC supply. After charging has completed, current drops to 62 ma. It looks like the net charging current is therefore 92 ma.
With the Data lines in any other configuration - either unconnected, or tied high or low through a number of different resistor values - the total current is only 114 ma. Since the new standard requires the downstream device to negotiate permission to draw more than 100 ma., perhaps the Clip refuses to draw more than that from what *might* be a normal USB port.
In any case, it appears the net charging current drops from 92 ma. to 52 ma. if the Data lines are not shorted, which would roughly double the time required to charge the batteries. But even a non-compliant charger would still charge the Clip overnight. So if someone gets no significant charging at all through a USB charger, it may well be because Auto Power or Sleep is turning charging off after 10 minutes, or whatever the setting is.
Hope this is helpful.