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This issue is more with the USB specs. USB Host ports make available either 100 mA or 500 mA of power at 5.5 v. The X50 standard battery is 1100 mAh. If you get 3 hours of use from it, then the X50 is drawing about 400 mA just to run. Not much extra current available to start with.
Now add the extra power you need to charge the battery. Note that the Dell mains power supply is rated at 2400 mA. It's that robust for a reason: the charging circuit will draw a LOT of power to charge the battery when it is significantly depleted. I've noted that when the battery is heavily depleted even the mains power unit runs warm, indicating to me that the draw is pretty high. The charging circuit in the Ax kicks in to charge the battery whenever the battery is below 100% and 5 v or so is available at the port. If the battery is more than about 80%, and if you keep the screen brightness on external power down at a lower level, and if you don't run BT/WiFI/hard drives/GPS/etc, then a USB port can keep up, barely. Below about 80%, or with heavier usage by bright screens, etc, the USB port runs out of oomph, and voltage drops as the current draw exceeds its ability to support. Two things happen: First, the Ax switches to battery and back to external as the USB port drops in voltage and recovers when the Ax quits drawing power and secondly the charging circuit starts to draw the power it needs any time 5v is present at the port EVEN IF THE CURRENT SUPPLY IS INADEQUATE. If the supply is inadequate, the circuit will start to draw from the battery to make up the shortfall, in essence charging the battery FROM the battery. The net effect is that the battery discharges even quicker plugged in than it does disconnected. The alternating accept/reject external power is what makes the screen flicker--if you have the brighness set differently for the two options.
So, while you can keep a battery charged and run the X50 from a USB port, it's going to be dicey, no matter who makes the cable.
For better success, use the USB cable to charge the battery when it is above 80%, keep the X50 OFF, or dim the screen as much as you can stand, don't use wifi or BT and don't use things like hard drives and GPS units.
The same logic applies to car chargers. Make sure the charger you get supplies 5.5v, not 5v, at 850 mA, not 350 or even 600. The lesser units simply won't be able to keep up with the demand.
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Want to know about GPS? See GPS 101. Want to know more? See GPS 201.
We must pay more careful attention, therefore, to what we have heard, so that we do not drift away. Hebrews 2:1 (NIV)
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. By the time this is over I will have spent nearly $13 on shipping charges. What a waste.