No charge while using GPS app...

I'm using the MotionX-GPS app on a iPhone 3G. On a 3 hour road trip recently, I connected the phone to the car charger cable and proceeded on my trip. I noticed that even though the phone was on the charge cord, the battery was still depleting at a good pace. It finally used up the phone battery about 1 hour into the trip, and I had to turn off the GPS function. To keep the phone display from turning off, I disabled the autolock function to force the screen display to be on all the time. Why wouldn't the phone maintain a battery charge while on the auto charge cord? Was the power drain more than the charger could keep up with? This problem keeps me from using my iPhone as a GPS nav device. On the trip back, I only used the GPS app as needed, and turned it off to let the phone charge back up.