This paper utilizes GPS tracked multiday travel activities to estimate the temporal distribution of electricity loads and assess battery electric vehicle (BEV) grid impacts at a significant market penetration level. The BEV load and non-PEV load vary by time of the day and day of the week. We consider two charging preferences: home priority assumes BEV drivers prefer charging at home and would not charge at public charging stations unless the state of charge (SOC) of the battery is not sufficient to cover the way back to home; and charging priority does not require drivers to defer charging to home and assumes drivers will utilize the first available charging opportunity. Both home and charging priority scenarios show an evening peak demand. Charging priority scenario also shows a morning peak on weekdays, possibly due to workplace charging. Assuming a significant percentage of the vehicle population in Seattle is displaced by BEVs, the BEV electricity demand is added to the non-PEV load. The critical BEV market shares are computed, under different charging assumption, beyond which the total electricity demand might exceed the generation capacity during the peak hour. Results show that the current power grid in Seattle can accommodate charging demand for about 80 thousand BEVs, which account of about 20% of total vehicle population.