It doesn't charge too fast. Actually once in the past using a Verizon phone, I can't remember I think it was a Droid X, and plugged in to THEIR car charger it still lost charge. I was running music on it, and utilizing the GPS while driving. The phone was quite hot and just barely able to maintain the charge, and honestly like I said I think it lost charge. That phone's GPS really seemed to work on it. I find that my Samsung Galaxy SII skyrocket doesn't charge very fast. In fact it seems to make a difference to disable the screen, wifi, and even kill the programs with the task manager.