I have an HTC Desire, which I frequently use as a Sat Nav system. I have a car charger, but this is often barely able to keep pace with the power draw of the phone. This seemed strange as the official HTC wall-wart charger that came with the phone can fully-charger the battery quite quickly. I did some research and discovered that the HTC wall charger has the USB data pins (pins 2 and 3) shorted. The phone detects this and increases the total power it draws whilst charging.
I believe that other phones may also use a similar system. On the Desire it is possible to check which kind of charge mode the phone is using by going to Menu: Settings: About Phone: Battery. The battery status can be displayed as either Charging (USB) or Charging (AC)
The fix to the slow charging issue, the usb data pins need to be shorted, either in the cable (not recommended if you want to use it as a data cable) or in the car charger itself. I couldn’t see any downside to the latter so that is the route I took. I should also point out at this point that I got a car charger which specified at least 1amp output. Chargers specced for 2amp output are widely available now and I would recommend getting one.
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