This is because USB connection typically need to electronic "negotiate" the amount of power they can safely get. At first, electric current is 100 milliamps, but after some 2-way communications over the USB cable, electric current can be re-negotiated to 500 milliamps.
The car charger and the wall charger contains this intelligence (It's as if a USB driver is pre-installed inside the chargers). That's why a driver must be installed in order to function.
If I buy a generic USB AC adaptor for the BlackBerry, it charges slooooow because many generic USB AC adaptors don't have the USB power negotiation. The RIM AC adaptor and the RIM Car Adaptor does have this feature, though.
In addition to this, many computers and laptop automatically shut off power on an inactive USB port, to save electricity, unless something is plugged into it, then 100 milliamps will commence without permission, but the USB device must electronically "ask for permission" via negotiation over the USB cable, to get more power.
In many cases, the BlackBerry might still charge from a partially inactive USB port (supplying 100 milliamps), but it will take a lot longer to charge -- sometimes several hours before you see the battery bar move from 1 notch to 2 notch. Other times, not at all -- especially if it's not even getting the 100 milliamps at all.
Other USB devices are able to draw 500mA power anyway even without negotiating it for first - it would typically be done with a generic USB adaptor that didn't care how much power was given out up to 500 milliamps.
Try charging your BlackBerry again, but this time, leave it connected for a much longer period. It might work.
Originally Posted by tkodet
Why can the car charger and wall charger both charge the BB via the USB port, but the Sync/Charging cable must be plugged into an "active" USB port??
I have an APC laptop bag which plugs into the wall, car, and airplane. It charges my laptop and 2 USB devices. Every other device's Sync/Charging cable has charged the device off of these "power only" ports. Not the BB. . .
If you are a road warrior and haven't seen these bags check them out (they aren't new). They can save you from carrying a TON of different adapters!!www.apc.com