Obviously, a phone simultaneously running 3G, bluetooth, GPS, & playing music uses a lot more power than a sleeping phone running no services lying on the seat. I soon discovered that my off-the-shelf lighter-plug car charger would not provide sufficient current to keep up. Even when only using the GPS my phone's battery was draining. After several purchases of progressively more expensive chargers I finally broke down and did some R&D.
I've since built a multi-port linear supply from scratch into the dash of my truck, but my wife wanted a no-dash-cutting solution for her car.
...more on the bigger linear supply charger in a separate instructable later...
Step 1: Background
To simplify the jargon and only address what's necessary for this article: The USB standard provides for a non-data charging connection that will provide higher current. You can get the full rundown at wikipedia
I'm going to disclaim here for a second: I'm not responsible if you melt down, electrocute, or violently explode your very expensive device, your car, your home, or your dog. These are all very real possibilities if you misuse or make any alterations to electrical devices. Use sense and caution when voiding any warranty.
That being said, the method that devices use to determine when they can charge at a higher current is through the USB connection itself. A standard USB type-A connector uses 4 pins: The 2 outside pins are for supplying current and the 2 center pins are for data. If the center 2 data pins are shorted together then it is obvious that no data can flow. This is an indication to the device that it can charge at a faster rate.
Step 2: Mod
Once you have the charger in hand the mod is simply this: open it up, solder the center 2 pins together, and put it back together.