To anticipate the impacts of electric vehicle (EV) charging on grid systems and the effectiveness of demand response measures for load control, it is critical to understand the determinants of EV charging demand. Previous research suggests that these determinants include both observable patent metrics of travel demand and less easily measurable triggers of charging decisions (such as range appraisal or habit). Nevertheless, analyses accounting simultaneously for both aspects are lacking. Data are used from a survey administered to EV drivers participating in the Low Carbon London EV trial to explore charging decision triggers to test their predictive power of observable metrics of charging demand, while controlling for variability in travel patterns. Results show that charging demand is significantly affected by travel pattern metrics as well as charging decision triggers.